Mid to Late-Holocene environmental dynamics recorded in Lake Pup Lagoon,East Antarctica: Insights from environmental magnetism and biogeochemical proxies

Abstract

Paleoenvironmental archives in East Antarctica have revealed significant changes during the Holocene, marked by ice sheet retreat leading to the isolation of submarine basins. These basins offer valuable insights into past climate, glaciology, and oceanography shifts that impact sedimentary processes. In this study, environmental magnetism and biogeochemical proxies to investigate the Mid-to-Late-Holocene transitions in Pup Lagoon, a coastal isolation basin is presented. Our findings reveal distinct stratigraphic zones reflecting shifts from marine to lacustrine environments. The results reveal predominant mechanical weathering in the Stornes region, producing coarse-grained “soft” ferrimagnetic minerals. Notably, a period of warm oceanographic conditions between 6000 and 4722 cal. yr BP was characterized by mixed magnetic grain sizes and ultrafine superparamagnetic grains, indicating relatively oxic open waters in the basin. Subsequent shifts to reducing conditions coincide with persistent marine sea ice cover from 4722 to 2634 cal. yr BP, favoring the retention of coarse-grained ferrimagnets. Finer magnetic grain sizes between 2634 and 2185 cal. yr BP was attributed to the increased freshwater inputs associated with the Mid-Holocene Hypsithermal. Further, diagenetic changes under persistent sea ice cover between 2185 and 1970 cal. yr BP led to the selective dissolution of fine-grained ferrimagnets. Transitioning to freshwater isolated basin conditions between 1970 and 588 cal. yr BP, fine ferrimagnet precipitation indicate oxic to suboxic conditions alongside drier conditions. Biogenic productivity increased post-isolation, which was reflected in increased (Total Organic Carbon) TOC and (Total Nitrogen) TN percentages. Additionally, the presence of greigite in the isolated phase sediment indicates reducing conditions owing to organic matter decomposition. Notably, χ_fd% exhibits an inverse trend to sea ice concentration, potentially indicating anoxic-dysoxic conditions due to the presence of sea ice. These observations align with broader regional sea ice concentration changes, emphasizing the interconnected behavior of local and regional factors shaping Antarctic coastal environments.

Keywords

East Antarctica lakes environmental magnetism Holocene sediment isolation basin

Introduction

The Antarctic ice sheet profoundly influences Earth’s systems, regulating climate and global sea level, as well as ocean temperature, circulation, and terrestrial environments (Flower and Kennett, 1994; Jones et al., 2022). Through a complex interplay of processes and feedback mechanisms, including alterations in ice elevation, planetary albedo, and oceanic and atmospheric circulation, the ice sheet affects various Earth subsystems, both directly and indirectly (Fyke et al., 2018; Sproson et al., 2021; White et al., 2022). These interactions gave rise to intricate weathering patterns, sediment transport mechanisms, aquatic biological productivity, and glacio-isostatic landmass shifts along coastal areas, with the ice sheet’s advance and retreat throughout the Holocene leaving a profound imprint (Johnson et al., 2022). Notably, abrupt Mid-Holocene ice sheet loss events, triggered by atmospheric and oceanic forcings, have led to significant ice shelf collapses (Fogwill et al., 2014; Hillenbrand et al., 2017; Johnson et al., 2019; Noble et al., 2020). Given the pivotal role of the Antarctic ice sheet, studying coastal and nearshore aquatic basins along the Antarctic continental margin assumes critical importance in unraveling these complex interactions through isostatic uplift, relative sea-level changes, retreat of glaciers, sea-ice changes, and colonization of newly formed biomes (Denis et al., 2010; Gao et al., 2020; Stutz et al., 2021; Verleyen et al., 2011).

Research efforts have increasingly turned to environmental proxies to elucidate sedimentation patterns and paleoenvironmental dynamics in coastal ice-free regions of Antarctica, particularly in basins transitioning from marine to lacustrine environments. Such studies have unveiled significant paleolimnological changes, including shifts in sediment composition and ecology, due to the impacts of changes in glaciological oceanographic conditions, as evidenced by various proxies (Hodgson et al., 2009; Matsumoto et al., 2010, 2014; Sterken et al., 2012; Takano et al., 2012, 2010; Verleyen et al., 2004a, 2004b; Zwartz et al., 1998). Environmental magnetic techniques, in particular, offer considerable promise in this domain. These techniques can discern alterations in iron-bearing minerals formed under distinct environmental conditions, shedding light on factors such as weathering, erosion, catchment geology, transport mechanisms, deposition processes, and post-depositional alterations. Consequently, changes in the nature, size, and concentration of magnetic minerals in sediments furnish crucial insights into past transformations within natural systems (Ao et al., 2016; Basavaiah et al., 2015; Balascio et al., 2011; Evans and Heller, 2003; Wang et al., 2016, 2022). Indeed, magnetic methods have been widely employed in reconstructing paleoenvironments across glacial-interglacial cycles in Antarctica’s climatic history (Chaparro et al., 2017; Joju et al., 2024; Noronha-D’Mello et al., 2021; Ohneiser et al., 2015; Phartiyal, 2014; Sagnotti et al., 2001; Warrier et al., 2014, 2021a). Additionally, proxies such as carbon and nitrogen content in sediment offer valuable insights into organic matter types and quantities within lakes, reflecting biological productivity responses to climatic variables, ice cover, and terrestrial processes (Mahesh et al., 2015).

The Larsemann Hills, a coastal oasis characterized by low-lying peninsulas and numerous lakes, occupies an ideal position between the ice sheet and Prydz Bay. This positioning exposes it to a combination of cryospheric, terrestrial, and oceanographic influences, which have varied over time, thereby significantly impacting coastal aquatic systems. The recorded variations in the ice sheet retreat since the LGM have led to varying responses among the lakes due to their geomorphic positions within the region (Gillieson et al., 1990; Hodgson et al., 2001; Noronha-D’Mello et al., 2023; Sabbe et al., 2004; Verleyen et al., 2004a, 2004b). Recent studies utilizing lake sediment archives from the Larsemann Hills have shed light on ice sheet dynamics, biological productivity, and relative sea-level changes, highlighting diachronous ice sheet retreat influenced by local geomorphological factors (Gao et al., 2020; Hodgson et al., 2001, 2006, 2009, 2016; Mahesh et al., 2018, 2021a, 2021b, 2022; Noronha-D’Mello et al., 2021; Verleyen et al., 2004a, 2004b, 2005, 2011).

Furthermore, isolated basins within the Larsemann Hills, which have shifted from marine to lacustrine environments, provide valuable insights into regional glacio-hydro isostasy changes (Hodgson et al., 2001, 2009; Verleyen et al., 2004a, 2004b). One such basin is the Pup Lagoon, a former marine relict lake situated along the coastline of the Larsemann Hills, which underwent isolation from the marine environment during the Late-Holocene, attributed to glacial-isostatic uplift. Previous research in the Pup Lagoon, exemplified by Verleyen et al. (2004a, 2004b), has provided compelling evidence of Pup Lagoon’s isolation as a freshwater lake and offered insights into past coastal oceanographic conditions in Prydz Bay through diatom assemblages. Gillieson et al. (1990) conducted hydrological and sedimentological analyses of Pup Lagoon, revealing that the lake is significantly influenced by sea spray. Their research indicates that the current coastline at Pup Lagoon was exposed by 4500 years BP based on a basal ¹⁴C age. Sabbe et al. (2004) investigated the limnological diversity in Pup Lagoon and other lakes in the Larsemann Hills and found that coastal lakes like Pup Lagoon have high accumulation rates due to elevated annual production and nutrient availability, promoting microbial mat production. Additionally, Mahesh et al. (2021a) conducted sediment grain size texture analysis in Pup Lagoon, shedding light on changes in coastal hydrodynamic conditions. Expanding upon this foundational research, the analysis of environmental magnetism, carbon, and nitrogen content of Pup Lagoon sediments could provide information that will help elucidate lake catchment conditions, sediment source, depositional conditions, and post-depositional processes (Phartiyal, 2014). Thus, Pup Lagoon holds considerable significance for paleoclimatological and paleoenvironmental inquiries, providing valuable insights into how isolated lakes respond to environmental changes. In this study, we aim to assess changes in environmental magnetism, carbon, and nitrogen content to understand the lake’s development in response to evolving marine and terrestrial environmental conditions during Mid to Late-Holocene climate variations, thereby providing a broader understanding of lake basin and hinterland changes over time.

Geomorphological settings of the study area

The Larsemann Hills (69° 24′ S, 76° 13′ E) comprise an ice-free area within Princess Elizabeth Land in East Antarctica, covering approximately 50 km² along the southeastern shore of Prydz Bay (Figure 1a–c). This region features two main peninsulas, Stornes, Broknes, and minor peninsulas such as the Mirror Peninsula, as well as numerous islands in Prydz Bay, with over 150 lakes occupying local depressions formed as the continental ice sheet retreated (Gillieson et al., 1990; Hodgson et al., 2001; Verleyen et al., 2004b). Geological evidence suggests that while certain areas like Broknes lacked ice cover during the Last Glacial Maximum, others like Stornes retained ice sheet coverage until the Mid-Holocene (Hodgson et al., 2001). Beryllium-10 (¹⁰Be) exposure ages indicate that the Larsemann Hills became ice-free since the marine isotopic stage (MIS) 4, with an average glacial retreat rate of 71.57 m/kyr (Liang et al., 2020).

Figure 1.

(a) Map of the Antarctic continent with the study area in East Antarctica. (b) Location of Larsemann Hills in Princess Elizabeth Land, East Antarctica, situated east of the Amery Ice Shelf. (c) Location of Pup Lagoon on Stornes Peninsula within the Larsemann Hills. (d) Contour elevation map of the study area, with Pup Lagoon marked using a dotted line. A star symbolizes the core sampling location. Maps were created using Quantarctica from the Norwegian Polar Institute (Matsuoka et al., 2021).

The rocky Stornes Peninsula, situated west of the Larsemann Hills and projecting into Prydz Bay, features a relief of approximately 140 m and resides in a periglacial environment between the coast and the continental ice sheet. Composed of granulite facies pelitic gneiss and mafic granulite, Stornes has undergone complex deformational metamorphism. Deglaciation of Stornes has been reported to occur around 4500 cal. yr BP, with much of the coastline submerged in Prydz Bay during this period (Hodgson et al., 2001).

Pup Lagoon (69°24′26.38″ S and 76°03′21.12″ E), situated in the Stornes region, is a coastal shallow-water isolation basin located approximately 70 m from the shore at an elevation of 5 m above present sea level (Figure 1d). Covering an area of around 1 ha, it reaches a maximum depth of 4.6 m with a sill height of approximately 4 m (Gillieson et al., 1990; Mahesh et al., 2021a). The basin, classified as freshwater, is enclosed by Blundell granitic orthogneiss to the south and Johnston orthogneiss to the north (Carson and Grew, 2007). Pup Lagoon became isolated from the marine environment between 2200 and 2000 cal. kyr BP due to glacial-isostatic uplift (Verleyen et al., 2004b).

Materials and methods

During the 2017 austral summer, as a part of the 36th Indian Scientific Expedition to Antarctica, a 53 cm long core was retrieved from the Pup Lagoon from a water depth of 3 m. The sediment core (PL) was collected in an acrylic liner using a UWITEC piston corer that was mounted on a floating platform. Subsequently, the core was frozen at −20°C and shipped to the sample processing laboratory at the National Centre for Polar and Ocean Research (NCPOR), India. The core was logged for lithology, sub-sampled into 1 cm slices, and then lyophilized.

For chronological analysis, radiocarbon dating (¹⁴C) using an accelerator mass spectrometer (AMS) was carried out at the Poznań Radiocarbon Laboratory in Poland. We used the earlier reported ¹⁴C ages for the core (Mahesh et al., 2021a) and recalibrated the ages using the updated calibration curves (SHCal20 and Marine20). The new age model was based on five radiocarbon dates obtained from bulk organic material. No correction for reservoir effects was applied to the raw age (¹⁴C 465 yr BP) of the upper 0–1 cm section of the sediment core. However, a reservoir age correction of 1300 years (Berkman et al., 1998; Ingólfsson et al., 1998) was applied for radiocarbon dates at 10 cm and below, corresponding to ¹⁴C 3845 yr BP and earlier. The correction was applied in accordance with the prior investigation (Mahesh et al., 2021a), revealing marine diatoms from 51 to 12 cm, mixed assemblages (marine and freshwater) from 12 to 10 cm in the PL core, and predominantly freshwater diatom assemblages in the top 10 cm, suggesting an isolated basin. Furthermore, the ¹⁴C found in the surface sediment of freshwater lakes in the Larsemann Hills is known to be in a state of near-equilibrium with modern atmospheric CO₂ levels (Hodgson et al., 2001; Zwartz et al., 1998). The corrected ¹⁴C dates (yr BP) were then calibrated to calendar ages (calibrated years before the present: cal. yr BP) using the BACON 2.2 software based on Bayesian age-depth modeling (Blaauw and Christen, 2011). The age calibrations were made with the SHCal20 curve (Hogg et al., 2020) for the core-top age and the Marine20 curve for the deeper ¹⁴C dates (Heaton et al., 2020). Furthermore, the program calculated the sedimentation rate for each sediment sample depth. Sedimentation rates may not accurately reflect changes at the depths of dated samples and can vary with lithological variations.

Environmental magnetic measurements were performed on 51 bulk sediment sub-samples (2–53 cm, where the upper 2 cm was consumed during ¹⁴C dating) at the CSIR-National Institute of Oceanography (NIO), Goa. These sub-samples were carefully packed into plastic vials containing approximately 7 g of sediment. Magnetic susceptibility was measured at low (0.47 kHz, χ_lf) and high (4.7 kHz, χ_hf) frequencies on the 0.1 scale using a Bartington MS2B Susceptibility meter. Additionally, the frequency-dependent susceptibility (χ_fd%) expressed as a percentage was calculated using the equation χ_fd% = ((χ_lf− χ_hf)/χ_lf) × 100 (Dearing, 1999; Oldfield, 1991). The samples were subjected to an anhysteretic remanent magnetization (ARM) utilizing a 100 mT peak alternating field in combination with a constant DC bias field of 50 μT. This procedure was carried out using a Molspin AF demagnetizer equipped with an ARM attachment. The resulting ARM in the samples was quantified using an AGICO dual-speed spinner magnetometer. To estimate the ARM susceptibility (χ_ARM), the mass-specific ARM was divided by the applied DC bias field, following the methodology outlined by Walden et al. (1999).

Isothermal remanent magnetization (IRM) was induced on the sediment vials at incremental magnetic field strengths of 20, 60, 100, 300, 500, and 1000 mT, and reverse fields of −20, −100, −300, and −1000 mT using a Molspin Pulse magnetizer. The remanent magnetization of the magnetized samples was then estimated with the spinner magnetometer. The IRM recorded at a field strength of 1000 mT was designated the Saturation Isothermal Remanent Magnetization (SIRM). Magnetic mineral’s grain sizes were determined using the ratios χ_ARM/SIRM, χ_ARM/χ_lf, and SIRM/χ_lf. The coercivity of the iron oxide magnetic components was evaluated using the S-ratio (S-ratio = IRM_−300mT/SIRM) (Bloemendal et al., 1992; Evans and Heller, 2003). Moreover, the magnetic minerals were separated from selected samples using a handheld magnet and subjected to Scanning Electron Microscopy (SEM; JEOL JSM-6360 LV) operated at 15–20 KeV for images, together with an OXFORD INA 200- Energy Dispersive Spectrometer (EDS) for the semi-quantitative determination of the elemental content of magnetic particles.

Total carbon (TC) and total nitrogen (TN) percentages were quantified using a soil elemental analyzer (Thermo FLASH 2000 model) at the CSIR-National Institute of Oceanography, Goa, India. Total inorganic carbon (TIC) in bulk sediment samples was analyzed using a UIC coulometer (CM5017) and an acidification module (CM5330). The total organic carbon (TOC) percentage was calculated from the difference between TC and TIC. To identify zonation with age, the values of all analyzed parameters were entered into the paleontological statistics software package (PAST 4.03, Hammer and Harper, 2001) and analyzed using multivariate cluster analysis using Unweighted Pair Group Method with Arithmetic Mean (UPGMA) with Euclidian similarity index and stratigraphic constraints.

Results and discussions

Lithology and geochronology

The lithological profile, as described by Mahesh et al. (2021a), exhibits a consistent sequence of cyanobacterial mats followed by layers of silt, coarse sand, coarse silt mixed with clay, and fine silt and sand, with silt dominating throughout (Figure 2a). The sediment core exhibits distinct characteristics across various depth intervals. Greenish organic matter predominates at the surface layer from 0 to 10 cm. From 10 to 12 cm, the sediment transitions to a layer of fine-grained sand with a moderate presence of organic matter. Between 12 and 21 cm, the sediment comprises medium-grained sand with reduced organic matter and fine rock fragments. From 21 to 25 cm, the sand texture becomes coarser, accompanied by increased dark organic matter content and the appearance of small rock fragments. From 25 to 40 cm, the sediment maintains a coarse-grained sand texture with small rock fragments, diminished organic matter, and emitting a pungent odor suggesting anoxia. Finally, the lowermost layer, spanning from 40 to 53 cm, records fine-grained sand, interspersed with rock fragments and exhibiting less organic matter.

Figure 2.

(a) Lithology of the PL sediment core as documented by Mahesh et al. (2021a). (b) Age-depth model based on BACON 2.2. The red dotted line represents the best estimates for calibrated age, while the gray band (with gray dashed lines) signifies the 95% confidence interval, including upper and lower estimates. This model is derived from five ¹⁴C ages obtained from the core and supplemented by probability distribution curves of calibrated ages, providing information on the sedimentation rate. (c) Plot illustrating the variation of sedimentation rate with depth for the PL sediment core. (d) Depth distribution of sand, silt, and clay (Weight%) within the PL sediment core (Mahesh et al., 2021a).

Calibration of ¹⁴C ages provided revised chronological data (Table 1 and Figure 2b). The age-depth model reveals that the core spans the Mid-to-Late-Holocene period. The section at 40 cm dates to 4722 cal. yr BP, while the core’s base at 53 cm extrapolates to 5995 cal. yr BP. This timeframe captures key climatic events, including the Mid-Holocene deglaciation of the Stornes Peninsula and the subsequent decline in relative sea level in Prydz Bay (Hodgson et al., 2001, 2016). Earlier findings indicated that by 6000 yr BP, the ice sheet withdrawal exposed the peninsula’s edges, leaving a remnant ice dome that had a significant regional impact on the region (Hodgson et al., 2001). The sedimentation rate in the PL core ranges from 5.59 to 38.46 cm/kyr, averaging 14.2 cm/kyr, lower than the rates reported by Verleyen et al. (2004b) in Pup Lagoon, which varied from 17.89 to 169.38 cm/kyr, averaging 87.03 cm/kyr. This discrepancy likely arises from differences in coring locations. The sedimentation rate generally decreases from the near shore to the far shore of the lake, with the lowest rate occurring at the deepest part (Singh et al., 2008). Coarser particles tend to settle near the periphery, while finer particles migrate toward the central portion, resulting in this pattern. Sedimentation rates can vary significantly due to highly localized sediment sources and water depth variations within the lake (Squyres et al., 1991). However, despite these variations, the observed sedimentation rates in the PL core exhibit a consistent depth-related trend with the earlier sediment core collected by Verleyen et al. (2004b) (Figure 2c).

Table 1.

Details of AMS 14C dates for the Pup Lagoon sediment core, calibrated using BACON 2.2 (Blaauw and Christen, 2011). The Marine20 calibration curve was applied for ages older than 2000 ¹⁴C BP, while the SHcal20 curve was used for ages post-2000 ¹⁴C BP.

Depth (cm)	Lab ID	¹⁴C Age yr BP	Error	Calibrated age (Mean) cal. yr BP
0–1	Poz-101400	465	30	288
9–10	Poz-101401	3845	35	1913
20–21	Poz-101404	3880	30	2255
30–31	Poz-101426	4785	35	3193
40–41	Poz-101427	6000	40	4722
52–53	Extrapolated age			5995

Characterization of environmental magnetic properties, C and N content

The magnetic concentration-dependent parameters are denoted by χ_lf, χ_ARM, and SIRM and indicate variable concentrations of ferrimagnetic material. χ_lf represents the bulk concentration of magnetic minerals that include ferrimagnets such as magnetite, canted antiferromagnetic (haematite), paramagnetic (biotite), and diamagnetic components (organic matter) in natural samples (Phartiyal, 2014; Thompson and Oldfield, 1986; Walden et al., 1999; Warrier et al., 2014). χ_lf in the Pup Lagoon sediment core ranged from 1.90 to 21.68 × 10⁻⁸ m³ kg⁻¹ (avg. 10.89 ± 2.70 × 10⁻⁸ m³ kg⁻¹) (Table 2). Furthermore, χ_ARM is also a concentration-dependent parameter and biased toward the stable single domain (SSD) grain size (Maher, 1988) within the size range of 0.03–1 μm (Lin et al., 2019; Nolan et al., 1999; Youcha and Brachfeld, 2004). The χ_ARM values in the sediment core ranged between 0.05 and 0.71 × 10⁻⁵ m³ kg⁻¹ (avg. 0.16 ± 0.14 × 10⁻⁵ m³ kg⁻¹). The SIRM reflects the concentration of all remanence-carrying minerals (Oldfield, 1991; Walden et al., 1999). Correspondingly, the SIRM values displayed a range between 140.52 and 1706.52 × 10⁻⁵ Am² kg⁻¹ with an average of 393.75 ± 333.98 × 10⁻⁵ Am² kg⁻¹.

Table 2.

Environmental magnetic parameters, their average values, and standard deviations in the Pup Lagoon sediment core. Additionally, earlier reported values from Mochou Lake, Larsemann Hills, during the Mid-to-Late-Holocene (Noronha-D’Mello et al., 2021) are compared. Zonal averages for PL1–PL5 environmental magnetic parameters are also presented.

Sr. No.	Parameters	Units	Average ± Standard deviation in the PL core	Mochou Lake (Mirror Peninsula)	PL1	PL2	PL3	PL4	PL5
1	Mass-specific magnetic susceptibility: low frequency (χ_lf)	(10⁻⁸ m³ kg⁻¹)	10.89 ± 2.70	6.84	10.42	10.30	11.35	9.54	13.46
2	Susceptibility of anhysteretic remanent magnetization (χ_ARM)	(10⁻⁵ m³ kg⁻¹)	0.16 ± 0.14	0.06	0.10	0.10	0.18	0.07	0.41
3	Saturation Isothermal remanent magnetization IRM (SIRM)	(10⁻⁵ Am² kg⁻¹)	393.75 ± 333.98	101.61	274.68	241.16	407.90	181.05	999.70
4	Percentage of frequency-dependent susceptibility (χ_fd%)	%	4.80 ± 3.79	—	9.86	3.66	2.62	1.51	3.77
5	χ_ARM/SIRM	(A⁻¹ m)	39.18 ± 7.72	67.71	35.71	41.80	42.95	36.41	39.81
6	χ_ARM/χ_lf	—	14.47 ± 13.84	8.91	9.27	9.77	15.76	6.93	35.63
7	SIRM/χ_lf	(10⁻³ Am)	36.54 ± 34.25	15.47	26.32	23.36	35.45	19.28	89.36
8	S-ratio	—	0.96 ± 0.03	0.96	0.98	0.95	0.96	0.92	0.96
9	TOC	%	1.60 ± 3.31	—	0.34	0.39	1.09	0.20	7.05
10	TN	%	0.13 ± 0.30	—	0.008	0.011	0.104	0.000	0.645

The magnetic grain size of the sediments is determined by χ_fd% and the inter-parametric ratios of χ_ARM/SIRM, χ_ARM/χ_lf, and SIRM/χ_lf (Table 2). χ_fd% is responsive to the fine-grained particles around the Superparamagnetic- Single Domain (SP/SD) boundary (Worm, 1998). It specifically represents the presence of SP grains, particularly those falling within the size range of 0–0.035 μm, with a narrower range of 0.01–0.025 μm (Maher, 1988; Muxworthy and Williams, 2009). χ_fd% values <2% indicate no SP grains; values between 2% and 10% indicate a mixed assemblage of SP and coarse grains; and values >10% signify the predominance of SP grains (Dearing et al., 1997; Warrier et al., 2021b). The χ_fd% values in the Pup Lagoon varied between 0.39% and 11.67% (avg. of 4.80 ± 3.79%), suggesting a mixed assemblage of SP and coarse grains. The inter-parametric ratios of χ_ARM/SIRM and χ_ARM/χ_lf also reflect the magnetic grain size, where higher (lower) values of χ_ARM/SIRM and χ_ARM/χ_lf suggest a χ, the ratio of χ_ARM/SIRM exhibited values between 26.10 and 65.22 × 10⁻⁵ mA⁻¹ (avg. 39.18 ± 7.72 × 10⁻⁵ mA⁻¹), whereas χ_ARM/χ_lf values ranged between 4.51 and 94.01 (avg. 14.47 ± 13.84) suggesting considerable variations in magnetic grain size in the core. SIRM/χ_lf also serves as a magnetic grain size indicator, with higher (lower) values suggesting a coarse (finer) grain size (Oldfield, 1991; Thompson and Oldfield, 1986; Walden et al., 1999; Warrier et al., 2014). The SIRM/χ_lf values ranged between 15.26 and 242.73 × 10⁻³ Am⁻¹ (avg. 36.54 ± 34.25 × 10⁻³ Am⁻¹). The higher SIRM/χ_lf values (>70 × 10⁻³ Am⁻¹) suggest the presence of greigite, a ferrimagnetic mineral formed in situ due to reducing conditions. The scatter plot of χ_ARM/SIRM versus χ_fd% (Dearing et al., 1997) revealed that samples within the coarse SSD field indicate the control of SSD grains on the magnetic properties (Figure 3a), consistent with observations in lakes from the Schirmacher Oasis (Warrier et al., 2021b). A few samples plot in the region where there is a 50% contribution from SP grains. Furthermore, the biplot of χ_ARM versus χ_lf (Figure 3b; King et al., 1982), revealed that most magnetic minerals plot closer to the 0.2 μm slope line suggesting relative coarser grain size of the ferrimagnets. Smaller iron oxides, comprising both SP and SD particles, are often found embedded within host silicates and were also noted from the SEM-EDS images showing iron oxide minerals (ca. 10 μm) (Figure 4c). On the other hand, secondary ferrimagnetic minerals can form through various processes, including bacterial assimilatory and dissimilatory processes or products of early diagenesis (Chaparro et al., 2017; Lascu and Plank, 2013; Ludwig et al., 2013; Roberts et al., 2011). The biplot of SIRM versus χ_ARM/χ_lf indicates that the concentration of magnetic minerals decreases downcore as the size of magnetic grain size increases (Figure 3d).

Figure 3.

Bi-plots of magnetic parameters and inter-parametric ratios indicating the magnetic grain size and mineralogy of Pup Lagoon sediments. (a) Semi-quantitative magnetic granulometry plot of χ_ARM/SIRM versus χ_fd% (Dearing et al., 1997; Maher, 1988) for Pup Lagoon sediment zones PL1, PL2, PL3, PL4 and PL5. (b) Bi-plot of χ_lf versus χ_ARM (King et al., 1982). (c) Isothermal Remanent Magnetization (IRM) acquisition curves for selected samples of Pup Lagoon sediment, legend is provided horizontally above graph. (d) Scatter plot of SIRM versus χ_lf/χ_ARM (Logarithmic scale axes).

Figure 4.

Scanning Electron Microscopy (SEM) images of magnetic mineral grains extracted from the PL sediment core, accompanied by Energy Dispersive Spectrometer (EDS) data revealing the presence of magnetite in sediment at a depth of (a) 2 cm with relative percentages of Fe, O, C, and Si. (b) 21 cm with percentages of Fe, O, C, and Ca. (c) 41 cm with percentages of Fe, O, C and Si showing finer iron oxides.

The S-ratio provides insights into the iron oxide mineral composition, with values close to 1 indicating the presence of “soft” magnetic minerals, like magnetite, titanomagnetite, maghemite, or greigite, while values close to zero or negative suggest significant contributions from hematite and/or goethite (Bloemendal et al.,1992; Evans and Heller, 2003; Walden et al., 1999). The average S-ratio value for Pup Lagoon sediment is 0.96, suggesting the predominance of low coercivity minerals, but some samples show relatively lower values (0.87–0.92), signifying the presence of high coercivity minerals like hematite and goethite. IRM acquisition curves reach saturation at around 300 mT, indicating the dominance of ferrimagnetic minerals, particularly magnetite/titanomagnetite (Figure 3c). Furthermore, the results of environmental magnetic parameters are supported by SEM-EDS studies (Liu et al., 2007; Warrier et al., 2014, 2021a), and confirms magnetite (Fe₃O₄) as the primary carrier of magnetic remanence in Pup Lagoon sediments (Figure 4a and b). SEM-EDS analysis (2 cm depth/580 cal. yr BP) shows magnetite composition- Fe-33.1%, O-36.68%, and the presence of Al, Si, Na, Ca, and Cl indicates that the iron oxide minerals present in the sediment are land-derived (Williamson et al., 1998). Silicon in EDS spectra may result from diatom frustules adhering to the magnetic grains, while C could be attributed to organic matter. Magnetite (Fe- 34%, O- 31.63%) is also detected at 21 cm depth (2.26 cal. kyr BP) deposited during the marine phase. Bacterial magnetite is absent, as χ_ARM/SIRM and χ_ARM/χ_lf values consistently fall below the specified threshold limits of χ_ARM/SIRM > 200 × 10⁻⁵ mA⁻¹ and χ_ARM/χ_lf > 40 (Oldfield, 1999).

Sediment organic matter content provides insights into biogenic productivity and sediment composition. The TC values varied from 0.06 to 15.82% (average 1.60%), while TIC ranged from not detectable (“nd”) to 0.04% (average 0.01 ± 0.01%). TOC, calculated as the difference between TC and TIC, ranged from 0.04 to 15.82% (avg. 1.60 ± 3.32%). TN ranged from “nd” to 1.36%, (average 0.13 ± 0.3%). Higher TOC values suggest predominantly organic carbon in the lake sediment compared to inorganic carbon. Furthermore, the bulk organic matter (TOC) reflects mainly in-situ production of organic matter as previously reported δ¹³C values (−21.6 to −19.5‰) in the marine phase in Pup Lagoon (Hodgson et al., 2001) indicated organic matter from marine phytoplankton and algae whereas the δ¹³C values (−13.1 to −14.9‰) in the freshwater phase correspond to benthic microbial mats (Hodgson et al., 2009; Maslin and Swann, 2005). Also, despite low water column productivity, cyanobacteria, particularly benthic microbial mats, play a crucial role in organic matter production in Larsemann Hills lakes (Sabbe et al., 2004).

The magnetic concentration parameters (χ_lf, χ_ARM, and SIRM) within Pup Lagoon spanning the Mid to Late-Holocene were higher than in Mochou Lake, a coastal lake situated on the Mirror Peninsula within the Larsemann Hills region (Noronha-D’Mello et al., 2021). Lower χ_ARM/SIRM and higher SIRM/χ_lf suggest magnetically coarser sediments in Pup Lagoon. The elevated magnetic parameters imply increased iron oxide mineral influx, likely due to intensive glaciogenic erosion releasing coarse-grained magnetic minerals. Differences in parent rock material at Stornes contribute to variations in magnetic mineral concentration compared to Mirror Peninsula (Carson and Grew, 2007). The catchment rocks of the Pup Lagoon are composed of Blundell orthogneiss composed of garnet-bearing felsic orthogneiss, large k-feldspar megacrysts, and minor biotite whereas the Mochou Lake catchment is surrounded by Lake Ferris metapelite and Nella mafic gneiss. These differences in catchment rocks could have contributed to variations in magnetic characteristics. Furthermore, the average TOC content in Pup Lagoon (Table 2) was significantly lower compared to other lakes in the Larsemann Hills region, such as Progress Lake and Reid Lake- >20% (Hodgson et al., 2005, 2006) that, suggests a low level of biogenic productivity in Pup Lagoon probably associated with the transition from marine to lacustrine phase.

Temporal environmental variations in the Pup Lagoon core

Cluster analysis was employed on environmental magnetic, TOC, and TN data, resulting in the identification of five distinct stratigraphic zones (PL1–PL5) within the core (Figure 5k, Supplementary Figure S1). These zones were delineated based on characteristic downcore variations of the aforementioned parameters and correspond to different depositional environments, providing insights into the sedimentary processes occurring in the basin over time.

Figure 5.

Down-core variations of environmental magnetic parameters for the PL sediment core. (a) Magnetic susceptibility (χ_lf). (b) Susceptibility of anhysteric remanent magnetization (χ_ARM). (c) Saturated isothermal remanent magnetization (SIRM). (d) Frequency-dependent susceptibility (χ_fd%). (e) χ_ARM/SIRM. (f) χ_ARM/χ_lf. (g) SIRM/χ_lf. (h) S-ratio. (i) TOC%. (j) TN % in zones PL1, PL2, PL3, PL4 and PL5. (k) Cluster diagram of Unweighted Pair Group Method with Arithmetic Mean (UPGMA) with Euclidian similarity index and stratigraphic constraints. The black vertical dashed line represents the average, and the horizontal dotted line (Black) represents the boundary between each phase. The gray rectangle represents marine conditions in the basin, the blue rectangle represents the transition zone, and the white area represents freshwater conditions. Red horizontal arrows on the depth axis indicate the calibrated ages at respective depths.

PL1 (53–-1 cm; 6000–4722 cal. yr BP)

This stratigraphic zone extends its temporal span to ages preceding 4722 cal. yr BP, a period coinciding with the reported retreat of the ice sheet on the Stornes Peninsula (Hodgson et al., 2001) and the marine phase noted in the PL core (Mahesh et al., 2021a). The results of PL1 collectively provide evidence for the provenance of magnetic minerals derived through the mechanical weathering of rocks within the catchment. Subsequently, these minerals underwent in-situ alteration processes upon deposition in the Pup Lagoon basin. In PL1, the relatively lower values of χ_lf, χ_ARM, and SIRM indicate a low content of bulk iron oxide minerals (Figure 5a–g). Additionally, χ_ARM, χ_ARM/SIRM, and χ_ARM/χ_lf values were low, denoting a coarse magnetic grain size. However, the elevated χ_fd% values (9.64 to 11.67%) implying significant concentrations (>50%) of SP grains in the sediments (Figure 3a). Thus, the deposited magnetic minerals are composed of a mixed grain size assemblage. The S-ratio values of ca. 0.98 indicate the presence of “soft” magnetic minerals like magnetite and titanomagnetite (Figure 5h).

Generally, in Antarctic terrestrial environments, high χ_lf values and low values of χ_fd%, χ_ARM, χ_ARM/SIRM, and χ_ARM/χ_lf indicate cold (glacial) periods, marked by dominant frost/mechanical weathering releasing coarse-grained ferrimagnetic minerals that are transported by erosional agents to basins. Conversely, warm (interglacial) periods foster chemical weathering, forming fine-grained magnetic minerals in the SP and SD size range, resulting in low χ_lf values and high of χ_fd%, χ_ARM, χ_ARM/SIRM and χ_ARM/χ_lf (Reynolds and King, 1995; Ryu and Song, 2010; Warrier et al., 2014). However, in former submerged marine basins such as the Pup Lagoon, the environmental magnetic properties of the transported sediments have to be interpreted with caution. The magnetic mineral deposition process is influenced by water column characteristics, such as stratification, anoxia, and post-depositional processes like diagenesis. Under marine conditions, the dissolution of ferrimagnetic minerals may occur, resulting in a reduction in magnetic susceptibility (Sandgren et al., 1990). Hence, the diminished bulk magnetic concentration values and the presence of coarse magnetic grain size in this core section are contemporaneous with marine conditions (including the PL1 phase) reported to have occurred in the Pup Lagoon between 6109 and 2427 cal. yr BP (Mahesh et al., 2021a) and 5800 to 2000 cal. kyr BP (Verleyen et al., 2004b).

In the PL1 zone, the elevated χ_fd% suggests the presence of SP particles or those near the SP/SD boundary. This could indicate increased transport and better preservation of fine magnetic minerals under relatively oxygenated to mildly oxygen-depleted waters during the PL1 phase. Typically, fine magnetic minerals tend to dissolve under reducing conditions (Kawamura et al., 2007). The abundance of SP grains indicates their enhanced preservation under oxic to suboxic conditions. This stratigraphic zone aligns with the marine stratified and open-water conditions documented in the Pup Lagoon between 5800 and 5500 cal. yr BP (Verleyen et al., 2004b). The reported presence of abundant Chaetoceros resting pores, reduced sea-ice diatom content (low sea ice concentration), and lower levels of Bacteriochlorophyll pigments (decrease in anoxia) during this phase further supports this notion of less dissolution of fine magnetic minerals during the PL1 phase. The timing of this phase also coincided with the second Holocene climate optimum detected in ice cores (Masson et al., 2000). The SP grains observed in this zone may have originated from weak chemical weathering of exposed catchment rock under warm conditions. They could have been transported to the Pup Lagoon basin from ice-free regions through wind or meltwater transport. This scenario is consistent with the Holocene climate optimum, which suggests a warm climate conducive to generating finer grain sizes like SP grains through chemical weathering processes. Furthermore, in the arid Antarctic environments, SP grains are also generated through glaciogenic processes such as rock abrasion by the clasts carried in the basal glacier ice. Alternatively, aeolian transport of the magnetic grains could also lead to the formation of SP grains via saltation, during which the grains are rounded, sorted, and reduced in size by grain-on-grain collisions during their transport by saltation (Ohneiser et al., 2015). Given the substantial separation between glaciers and the Pup Lagoon, it is probable that the aeolian transport mechanism could have also played a substantial role in generating SP grains. This is in line with the predominant movement of fine sediments resulting from erosion in the ice-free region of the Larsemann Hills, which are carried seaward by katabatic winds (Gasparon and Matschullat, 2006). Thus, relatively open water and oxic to suboxic conditions during the PL1 phase could have contributed to the enhanced preservation of SP grains and subsequently led to higher values. The low TOC and TN percentages in PL1 (zonal averages of 0.34% and 0.008%, respectively) in the entire marine phase indicate a period of low organic matter production likely sourced from marine phytoplankton or its low preservation (Figure 5i and j). Such low values of TOC were also observed during the marine phase of Stepped Lake (Mahesh et al., 2018; Verleyen et al., 2004a). The high percentage of silt and sand (Figure 2d) in this section could result from glacio-fluvial deposition in the basin (Mahesh et al., 2021a).

PL2 (41–26 cm; 4722–2634 cal. yr BP)

PL2 spanned from 4722 to 2634 cal. yr BP and exhibited a low magnetic concentration and coarse magnetic grain size (Figure 5a–g) similar to PL1. However, the χ_fd% values were low in this zone, ranging from <2% to 6%, largely indicating the presence of the mixed assemblage of SP and coarse grain size. The plot of χ_ARM/SIRM and χ_fd% also exhibited a reduction in SP grains (Figure 3a). The S-ratio values in PL2 were lower than those in PL1 (Figure 5h), with a zonal average of 0.95, indicating a reduced presence of soft ferrimagnets (Table 2). The decline in the presence of SP grains and soft ferrimagnets in PL2 indicate strong reducing conditions in the basin, which resulted in their increased dissolution and subsequent predominance of coarse-grained ferrimagnets in the sediments. These changes are likely a consequence of increased basin-wide ice coverage, reduced wind mixing, and fostering anoxic to dysoxic conditions, ultimately leading to the dissolution of fine ferrimagnets and magnetic grain coarsening (Figure 6h). This was also supported by an increase in bacteriochlorophyll pigments, reported between 5500 and 2750 cal. kyr BP, suggesting the presence of anaerobic bacteria and sediment anoxia due to ice cover during this time (Verleyen et al., 2004b). The detrital magnetite material, under anoxic conditions, would have undergone reductive dissolution, thereby weakening the magnetic susceptibility (Balascio et al., 2011). Such selective dissolution of fine-grained iron-oxide minerals could have contributed to a decrease in magnetization intensity and an increase in the coarseness of the magnetic fraction (Pérez-Cruz and Urrutia-Fucugauchi, 2018). The PL2 zone is interpreted to undergo anoxia and reducing conditions due to the increasing sea ice cover.

Figure 6.

Downcore variation of (a) magnetic susceptibility (χ_lf). (b) SIRM. (c) χ_ARM/χ_lf. (d) χ_ARM/SIRM. (e) TOC. (f) Mean grain size (MGS, Mahesh et al., 2021a). (g) χ_fd% in the Marine phase (0–12 cm, 5995–1987 cal. kyr BP). (h) Sea Ice diatom group (%) – Pup Lagoon-, Fragilariopsis curta, Fragilariopsis angulata, Fragilariopsis cylindrus, Fragilariopsis curta, Fragilariopsis sublinearis, Navicula glaciei, Navicula perminuta, Entomoneis. Kjellmanii, Navicula sp., Nitzschia sp. (Verleyen et al., 2004b) indicating the concentration of sea ice; Ages have been recalibrated using Marine20 and SHcal20 ¹⁴C calibration curves (Heaton et al., 2020; Hogg et al., 2020). (i) Sea Ice diatom group (%) for Heart Lake (Verleyen et al., 2004a). (j) Sea Ice diatom group (%) in Prydz Bay (Denis et al., 2010).

PL3 (26–20 cm; 2634–2185 cal. yr BP)

A minor increase in χ_lf, χ_ARM, and SIRM, as well as in magnetic grain size parameters χ_ARM/SIRM, χ_ARM/χ_lf, and SIRM/χ_lf, indicate an increase in the concentration of ferrimagnets and a shift toward finer magnetic grain size (Figure 5a–g). The samples of PL3 also fell within the coarse SSD range, with relative grain sizes <0.2 μm (Figure 3a and b). Furthermore, χ_fd% exhibited values lower than the average (2–5%). The increase in magnetic concentration parameters and finer grain size could indicate a redox peak, suggesting the precipitation of finer iron oxides at the redox boundary. The increase in S-ratio values toward 1 also supports the precipitation of low coercivity minerals (Figure 5h). This trend may be attributed to relatively oxic conditions in the basin, likely associated with reduced winter sea ice cover and a prolonged period of open water conditions in the basin (Figure 6h). This inference is coeval with the reduced/thinner sea ice cover and relatively oxic conditions between 2750 and 2200 cal. yr BP, inferred from the decreased sea-ice diatoms and bacteriochlorophyll levels (Verleyen et al., 2004b). Furthermore, the increase in sedimentation rate (Figure 2c) suggests increased inflow and retention of freshwater in the basin during this phase, which may have induced brackish conditions. These conditions could have facilitated the preservation of fine ferrimagnets. This zone corresponds to the Mid-Holocene Hypsithermal period, characterized by warmer climatic conditions, which likely led to the melting of accumulated snow and ice in the catchment area. Additionally, the thinning of sea-ice cover could have also supported high organic matter production in the basin (Obryk et al., 2016), resulting in a minor increase in TOC and TN (Figure 5i and j).

PL4 (20–12 cm; 2185–1970 cal. yr BP)

Zone PL4 was characterized by a decrease in χ_lf, χ_ARM, and SIRM values, implying reductions in magnetic minerals. The magnetic granulometric parameters χ_fd% (<2%), χ_ARM/SIRM, χ_ARM/χ_lf, and SIRM/χ_lf also decreased, implying depletion of fine-grained magnetic minerals or selective dissolution of fine magnetite (Figure 5a–h). The anomalous decrease in the S-ratio to 0.87 suggests a mixed mineralogy comprising both soft and hard components (Bloemendal et al., 1992; Phartiyal, 2014). This low S-ratio indicates a substantial concentration of high-coercivity magnetic minerals such as hematite and geothite, along with paramagnetic minerals such as biotite (Sandgren and Snowball, 2002). The decline in S-ratio in this zone indicates the selective dissolution of magnetite relative to hematite. Moreover, the samples of PL4 also fall in the coarse SSD region of the plot of χ_ARM/SIRM versus χ_fd% with relative grain size >5 μm (Figure 3a and b), indicating the progressive dissolution of fine magnetic grains. Studies by Kawamura et al. (2007) suggest that magnetite undergoes dissolution as it traverses the iron redox boundary, particularly within the suboxic zone, during early diagenesis. Similarly, it could be possible that the fine-grained magnetic minerals within the sediments in this zone might have undergone rapid dissolution due to the effects of early diagenesis and anoxia due to persistent sea ice cover. This zone corresponds to the higher percentage of sea-ice diatoms and bacteriochlorophylls noted between 2200 to 2000 cal. yr BP in the preceding study, which suggests extensive ice coverage over the lake and the presence of anoxic conditions (Figure 6h). Moreover, TOC and TN percentages also decreased within this zone, suggesting reduced productivity due to ice cover. Despite the increased ice cover, the sedimentation rate peaked in the core in this zone, ranging from 34.48 to 38.46 cm/kyr, coinciding with a high sand percentage (Figure 2c and d). This substantial influx of inorganic material suggests an ice-proximal setting during the transition, likely originating from a local ice mass formed by the accumulation of snow and ice in the catchment of the basin, as also reported in the Kirisjes pond in the Larsemann Hills region (Hodgson et al., 2009). However, the lake’s elevation would have likely stabilized at 0 m above sea level after its isolation from the marine system, potentially resulting in the lake basin being impacted by sea ice, facilitating the deposition of entrapped sediments onto the shoreline and into the Pup Lagoon (Mahesh et al., 2021a).

PL5 (12–2 cm; 1970–581 cal. yr BP)

The PL5 zone corresponds to the diatom transition zone observed between 12 and 10 cm, delineated by a mixed assemblage of marine and freshwater diatoms. This transition leads into the isolated basin phase between 10 and 2 cm, characterized by freshwater diatom assemblages predominantly consisting of Stauroforma inermis, as noted in the PL core by Mahesh et al. (2021b). In the transition zone (12–10 cm, 1970 to 1913 cal. kyr BP), a steep increase in χ_lf, χ_ARM, and SIRM values, and χ_ARM/ SIRM, χ_ARM/χ_lf and SIRM/χ_lf is observed (Figure 5a–g). These values exhibited a significant contrast with those of the other zones (Table 2), suggesting increased magnetic mineral concentration and finer magnetic grain size, relatively ranging from <0.1 μm (Figure 3b). The S-ratios also remained consistently close to 1.00, suggesting dominance of ferrimagnets (Figure 5h). The environmental setting between 10 and 12 cm in the lake is linked to the transition of the Pup Lagoon basin from the marine regime, occurring ca. 2000 cal. yr BP (Verleyen et al., 2004b). The abrupt rise in magnetic parameters may suggest the precipitation of fine ferrimagnets at the redox boundary upon encountering relatively oxic to suboxic conditions. Additionally, the increased magnetic concentration and finer grain size could be attributed to the transport of coarse and fine magnetic minerals facilitated by intermittent freshwater inputs during relatively drier conditions associated with Neoglacial cooling. Similar observations correlating high magnetic susceptibility to basin isolation were noted by Sandgren et al. (1990) in Lake Adran.

The basin was isolated from the marine regime in the interval spanning 10–2 cm (1913 to 588 cal. yr BP). During this phase, χ_lf, χ_ARM, and SIRM demonstrate a declining trend, alongside χ_ARM/SIRM, χ_ARM/χ_lf, and SIRM/χ_lf, suggesting a decrease in magnetic concentration and fine magnetic grain size. The lake ice cover could have existed as a seasonal ice moat after the basin isolation. The preceding study inferred the presence of bacteriochlorophylls and a monospecific freshwater diatom assemblage observed post-2000 cal. BP that suggested oxygen depletion, consistent with ice cover (Verleyen et al., 2004b). Furthermore, high SIRM/χ_lf values (>70 × 10⁻³ Am⁻¹) in this zone suggest the presence of greigite in the sediment (Dash et al., 2022; Oldfield, 2007) that indicate its likely occurrence, while Oldfield (1999) reported values SIRM/χ_lf > 30 × 10³ Am⁻¹. These high SIRM/χ_lf values coincided with elevated percentages of TOC (4–16%) and TN (0.4–1.4%), suggesting that in-situ greigite production could be a result of high productivity (Warrier et al., 2021b). Greigite is an authigenic ferrimagnetic mineral formed through the breakdown of organic matter, driving the reduction of sulfates and indicating a reducing environment (Roberts, 1995). The decomposition of organic matter leads to reducing conditions under which Fe-oxide minerals are not stable. In arid regions such as Antarctica, where freshwater evaporation exceeds its replacement rate, the accumulated sulfate in the lake can react with iron, forming iron sulfides. Moreover, increased organic productivity within the lake system will likely dilute the magnetic signal and promote in-situ formation, leading to greigite formation (Warrier et al., 2021b).

A decreasing trend in χ_lf, χ_ARM, and SIRM values above 7 cm, as well as χ_fd%, χ_ARM/SIRM, χ_ARM/χ_lf, and SIRM/χ_lf ratios, are suggestive of a low magnetic concentration and finer magnetic grain size (Figure 5a–g). Notably, the surface of the core exhibited high values of χ_fd%, χ_ARM/SIRM, χ_ARM/χ_lf, and SIRM/χ_lf (Figure 5a–g), suggesting increasing finer magnetic grains due to weak pedogenesis in the region. These trends point largely toward terrestrial responses of environmental magnetic properties and indicate that the sediment deposition took place under predominantly warm environmental conditions (Wang et al., 2010; Warrier et al., 2014) post-1400 cal. kyr BP. Also, oxygen-depleted water conditions in the lake basin may have declined, leading to better preservation of fine magnetic grains, which is also evident from the decrease in bacteriochlorophylls (Verleyen et al., 2004b). Furthermore, the decrease in the χ_lf signal above 7 cm depth could be attributed to the abundance of organic matter (TOC and TN), reflecting compositional changes in the sediment (Domack and Ishman, 1992). This is supported by the elevated TOC and TN percentages, indicative of increased cyanobacterial mat production and/or better preservation of organic matter (Khim et al., 2004). This observation is also corroborated by the δ¹³C values falling within the range of −19.3 to 14.9‰ between 1.2 and 1.5 cal. kyr BP (Hodgson et al., 2001) indicates that organic matter primarily originates from freshwater algae sources. The sedimentation rate decreasing to approximately 5.59 cm/kyr (Figure 2c) indicates a decrease in the fluvial transport of sediment to the Pup Lagoon basin through the decrease of snow banks and ice in the catchment. The retreat of firnified snow and ice in the catchment of the lake facilitated soil-atmosphere interactions, leading to the production of fine ferrimagnets through weak pedogenesis, evident from the increased χ_fd% values toward the top of the section. The decrease in glaciofluvial deposits (sand) and the concurrent increase in silt and clay content (Figure 2c and d) imply the retreat of the local ice mass from the lake’s catchment area.

Comparison with other records

The basal age in the PL core (5995 cal. kyr BP) aligns closely with the previously reported onset of ice sheet recession on Stornes around 6000 years BP (Hodgson et al., 2001). However, recent data from the past decades suggest an earlier retreat of the ice sheet from the periphery of the peninsula. Carbon-14 dating results from a sediment core extracted from coastal lake LH4 on Stornes, exhibiting similar geomorphological features to Pup Lagoon, reveal a chronology dating back to 8990 ± 50 ¹⁴C yr BP (at 180 cm depth) and 8760 ± 50 ¹⁴C yr BP (at 160 cm depth) (Unpublished data). Kiernan et al. (2009) also noted a similar observation, proposing earlier deglaciation of Stornes based on ¹⁰Be exposure ages, suggesting the exposure from the ice occurred within 1–2 km of Stornes ice dome around the beginning of the Holocene approximately at 10 kyr BP. Liang et al. (2020) obtained ¹⁰Be exposure ages (20.9 ± 1.7 and 24.1 ± 2.1 kyr ) on the Stornes Peninsula, suggesting even earlier rock exposure due to glacial retreat. These studies collectively indicate a much earlier retreat of the continental ice sheet from the coast of Stornes. The basal ages obtained in Pup Lagoon (6 kyr, this study and Verleyen et al., 2004b) are notably younger than the exposure dates listed, which does not precisely indicate the presence of an ice sheet over the basin. This could be attributed to the coverage by a local ice mass that once covered the catchment and the submarine basin before 6 kyr, similar to conditions in adjacent lake LH22 (Figure 1d), which could serve as a modern analog. This suggests that the relatively younger basal ages noted in Pup Lagoon may be attributed to the retreat of accumulated snow and ice in the immediate catchment, which likely occurred around 6 kyr due to warm oceanographic conditions. Around 6 kyr BP, increased Southern Ocean subsurface water temperature due to higher summer insolation at high latitudes and a shift in the Southern Hemisphere Westerlies winds likely triggered ice melting along the Soya Coast, East Antarctica driven by amplified upwelling of Circumpolar Deep Water onto the Antarctic shelf (Crosta et al., 2018; Sproson et al., 2021; Voigt et al., 2015). Denis et al. (2010) also noted this process in Prydz Bay, potentially impacting coastal submarine basins such as the Pup Lagoon. Further, the coarser mean grain size of the basal 6 cm (53 to 45 cm) in the Pup Lagoon core (Mahesh et al., 2021a) and the presence of abundant rock fragments as compared to the section above (Figure 6f) suggests ice-proximal settings with glacial marine sedimentation would have occurred (Licht and Andrews, 2002). A similar basal unit of consolidated sediments composed of sand and grit was noted in the earlier study by Verleyen et al. (2004b) in Pup Lagoon. This could be attributed to the retreat of the local ice mass formed from firnified snow and ice from Pup Lagoon, following which sedimentation in the Pup Lagoon submarine basin may have commenced. Similar younger basal ¹⁴C ages, compared to other areas in the Broknes region, were also observed in Lake Nella, likely due to coverage by a remnant ice lobe (Hodgson et al., 2001). These results suggest that the Stornes region’s deglaciation might have occurred earlier. Further research based on ¹⁰Be exposure dates and sediment cores is necessary to determine the precise timing and pattern of ice sheet retreat on Stornes to constrain the onset of the last exposure process.

The sediments deposited in the Pup Lagoon provide insights for understanding diverse sedimentary processes under changing environmental conditions. They exhibit a robust signal of catchment soils through inputs of minerogenic and chemically weathered products. In warm climate conditions, chemical weathering of exposed rocks in the catchment generates fine ferrimagnets, which are subsequently transported to the basin via meltwater (Chapparro et al., 2017; Warrier et al., 2014, 2021a). Conversely, during cold climate periods, mechanical weathering predominates, resulting in the production of coarse-grained ferrimagnets and transport by aeolian means. Further, marine conditions in the basin can lead to decreased organic matter production, primarily from diatoms (Verleyen et al., 2004b), accompanied by the coarsening of magnetic grains (Abrajevitch and Kodama, 2011; Pérez-Cruz and Urrutia-Fucugauchi, 2018). As the basin transitions to a freshwater environment and becomes isolated, fine ferrimagnets are more likely to be preserved. Organic matter production shifts to primarily cyanobacterial mats (Sabbe et al., 2004). Moreover, the grain size of iron oxides varies with ice cover, resulting in oxygen-depleted conditions where finer magnetic grains undergo selective dissolution alongside changes in magnetic mineralogy (Kawamura et al., 2007). However, under sub-oxic to oxic conditions, finer magnetic minerals tend to be better preserved. Furthermore, ice cover influences the lake environment, with open water conditions associated with higher sedimentation rates and relatively greater diatom productivity characteristic of open waters (Hodgson et al., 2005, 2006, 2009; Obryk et al., 2016; Verleyen et al., 2004a, 2004b). Conversely, the presence of seasonal ice cover can restrict sedimentation and productivity in such basins.

The sediment influx into the Pup Lagoon lake basin results mainly from the catchment but undergoes post-deposition alterations due to changes in the redox conditions of the lake due to sea-ice cover, causing alterations in the environmental magnetic properties. Accordingly, the concentrations and grain size distribution of the contained magnetic minerals vary. The deposition of magnetic minerals during the lacustrine or isolated basin phase reflects terrestrial changes within the catchment area in response to drier conditions and is comparable to Neoglacial cooling that occurred ca. 2000 yr BP. The reduced meltwater flow due to arid conditions resulted in a decrease in sedimentation rate. Moreover, magnetic parameters and TOC (Figure 6a–e) display distinct responses, particularly during the marine phase. The χ_fd% parameter shows to be sensitive to increasing sea ice, as the dissolution of SP grains during oxygen-depleted conditions leads to a reduction in χ_fd% values (Figure 6g). This exhibits an inverse trend to sea-ice concentration in Pup Lagoon, as indicated by the abundance of sea-ice diatoms (Figure 6h). As sea-ice concentration increased, χ_fd% decreased, and vice versa. Thus, χ_fd% could potentially serve as a proxy for anoxic-dysoxic conditions, reflecting sea-ice conditions in marine basins. Additionally, the increase in sea ice concentration in Pup Lagoon reflects broader regional changes in the Prydz Bay region. The increasing sea-ice conditions in Pup Lagoon are contemporary with those observed in Heart Lake and Prydz Bay during the Mid to Late-Holocene (Figure 6i and j), suggesting the responses recorded in Pup Lagoon are part of a broader phenomenon. Furthermore, in contrast with the sea-ice diatom records from Prydz Bay (68°S), the coastal Pup Lagoon shows an earlier increase in sea ice concentration (post 5500 cal. yr BP), preceding the intensified sea-ice conditions observed in Prydz Bay post 4500 cal. yr. This occurrence is likely linked to the more enclosed location of Pup Lagoon on the coast of Prydz Bay. Additionally, the local morpho-bathymetry and proximity to the ice sheet could have facilitated cooling in the region and the persistence of sea ice. In contrast, the Prydz Bay system may be more influenced by broader climatic shifts, including the impact of the southerly position of the Southern Hemisphere westerly winds before 4.5 cal. kyr BP (Quade and Kaplan, 2017; Saunders et al., 2018; Voigt et al., 2015), oceanic current and atmospheric systems (Denis et al., 2010), and variations in oceanic heat flux (Lei et al., 2010); which can affect sea ice on its outer edges. Nevertheless, both locations exhibit an increasing trend in sea ice with intermittent periods of reduced sea ice conditions around the same time, reflecting changes in the sedimentary environment and magnetic properties in Pup Lagoon. This suggests an interconnected behaviour that regional environmental changes were likely influenced by larger-scale climatic phenomena, emphasizing the need for a comprehensive understanding of both local and regional factors shaping the Antarctic coast.

Conclusions

The magnetic properties of sediments in Pup Lagoon during the Mid to Late-Holocene were significantly influenced by magnetic mineral concentration, grain size, and composition variations. The higher values of magnetic concentration parameters (χ_lf, χ_ARM, and SIRM) in Pup Lagoon compared to Mochou Lake suggest an increased influx of iron oxide minerals, likely due to intensive glaciogenic erosion. Moreover, the presence of low coercivity minerals, indicated by the S-ratio values and IRM acquisition curves, supports the predominance of “soft” magnetic minerals in Pup Lagoon sediments. SEM-EDS studies further corroborate these findings, confirming magnetite as the primary carrier of magnetic remanence in the sediment. The low TOC and TN content are attributed to the transition from marine to freshwater settings.

Temporal environmental variations in the Pup Lagoon core demonstrate the dynamic interplay between depositional processes, redox conditions, and climatic fluctuations across different stratigraphic zones. Five distinct zones, each representing different depositional environments over time. Zone PL1 (6000–4722 cal. yr BP) indicates marine conditions with mixed magnetic grain sizes and ultrafine superparamagnetic grains, suggesting relatively oxic open waters probably associated with warm oceanographic conditions. In PL2 (4722–2634 cal. yr BP), marine sea-ice conditions persisted, causing reducing conditions favoring coarse-grained ferrimagnets. PL3 (2634–2185 cal. yr BP) shows finer magnetic grain sizes, likely due to increased freshwater inputs related to the Mid-Holocene Hypsithermal. PL4 (2185–1970 cal. yr BP) exhibits diagenetic changes under persistent sea ice cover, leading to selective dissolution of fine-grained magnetic minerals. PL5 (1970–588 cal. yr BP) exhibited transitions between marine and freshwater regimes, with greigite indicating reducing conditions and increased organic productivity during the Late-Holocene.

The observed variations in Pup Lagoon sedimentation, magnetic properties, and organic matter may be attributed to changes in sea-ice coverage, freshwater influx, and broader regional climate shifts. Particularly, χ_fd% shows an inverse trend to anoxia resulting from increased sea ice coverage over the basin during the marine phase. The interconnected behavior of environmental changes in Pup Lagoon and Prydz Bay suggests the influence of larger-scale climatic phenomena on Antarctic coastal conditions, highlighting the importance of considering local and regional factors in understanding past environmental dynamics.

The sedimentary records from Pup Lagoon provide valuable insights into environmental changes in the Antarctic coastal region. Comparison with earlier records suggests an earlier retreat of the ice sheet from the Stornes Peninsula, indicating complex deglaciation processes in the region.

Supplemental Material

sj-docx-1-hol-10.1177_09596836241266424 – Supplemental material for Mid to Late-Holocene environmental dynamics recorded in Lake Pup Lagoon, East Antarctica: Insights from environmental magnetism and biogeochemical proxies

Supplemental material, sj-docx-1-hol-10.1177_09596836241266424 for Mid to Late-Holocene environmental dynamics recorded in Lake Pup Lagoon, East Antarctica: Insights from environmental magnetism and biogeochemical proxies by Cheryl A. Noronha-D’Mello, Abhilash Nair, Mahesh B.S., Anish K. Warrier and Rahul Mohan in The Holocene

Footnotes

Acknowledgements

We extend our gratitude to the Director of the National Center for Polar and Ocean Research (NCPOR), Ministry of Earth Sciences (MoES), India, for their invaluable support under the Polar Science and Cryosphere Research (PACER) scheme. Additionally, we acknowledge the Director of CSIR-NIO for providing access to the Environmental Magnetic Measurement facility at the Paleomagnetic Laboratory, as well as for the analysis of TC and TN using the Elemental Analyzer within the Geological Oceanography Division and Central Analytical Facility of NIO. Special thanks to Dr. Pratima Kessarkar of CSIR-NIO for assisting with rock magnetic analysis and to Dr. Prakash Babu, Retired-CSIR-NIO, for supporting the elemental analyzer. We are grateful to the Norwegian Polar Institute for the Quantarctica software package. Furthermore, we acknowledge Ms. Sahina Ghazi for her contributions to the SEM-EDS analysis and Ms. Sati Chauhan for the coulometric analysis of TIC%. We also extend our appreciation to Prof. Elie Verleyen of Ghent University, Belgium, for providing the diatom microfossil datasets. Lastly, we sincerely thank the Indian Antarctic Program and the members of the 36th Indian Scientific Expedition to Antarctica for their invaluable support of this study. We also thank the editor and reviewers for their comments, which helped improve the manuscript’s quality. This is NCPOR Contibution number J-7/2024-25.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported under the PACER projects “Antarctic Science Co-ordination and Polar micropaleontology and Past Climate” by the National Centre for Polar and Ocean Research and the Ministry of Earth Sciences, Government of India.

Data Accessibility

The data supporting the findings of this study are available within the manuscript and its supplementary materials. Additionally, raw data and associated metadata are available upon request from the corresponding author (noronhacheryl@ymail.com).

ORCID iDs

Cheryl A Noronha-D’Mello

Anish K Warrier

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