Holocene changes in marine productivity and terrestrial organic carbon inputs into an Icelandic fjord: Application of molecular and bulk organic proxies

Abstract

This study examines the dynamics of organic carbon contributions from different sources to the sediments of a ~39 m core from Ísafjarðardjúp Fjord, Northwest Iceland, throughout the Holocene. Furthermore, it shows that the variability of terrestrial organic carbon (OC_terr) and marine organic carbon (OC_mar) is linked to palaeoclimatic change throughout the Holocene. glycerol-dialkyl-glycerol-tetraether (GDGT), alkenone, n-alkane, total OC and total nitrogen analyses were conducted on 326 samples to yield high-resolution branched versus isoprenoid tetraether index (BIT-index), n-alkane/alkenone index and C/N ratio records from ~10,800 to ~300 cal. a BP. These records were used to estimate the OC_terr and the OC_mar contributions to the sediments. Three different approaches of estimating the OC_terr contribution yield different relative amounts, but similar long-term trends. These results indicate that the combination of biomarker records is a good approach to reconstruct OC_terr contributions but also highlight the strengths and weaknesses of the individual biomarkers. The OC_terr contribution to the total OC inventory continually increases throughout much of the Holocene but does not rise above 30%. It seems to have been driven by changing climate rather than changing sedimentation rates, and during the late Holocene, anthropogenic activity may have been an influence. The reconstructed OC_mar contribution to the sediment was used to model changes in palaeoproductivity throughout the Holocene. These changes were likely forced by changes in nutrients supplied both by the catchment area and the Irminger Current.

Keywords

branched versus isoprenoid tetraether index C/N ratio Holocene Iceland palaeoproductivity

Introduction

Fjords play a major role in the global carbon cycle as storage reservoirs of organic carbon (OC), containing at least 12% of the total OC buried in the continental margin over the past 100,000 years (Nuwer and Keil, 2005). The generally high sedimentation rates prevalent in fjords are conducive to preserving OC (Smittenberg et al., 2004). Additionally, the seasonal or permanent anoxic conditions prevalent in the water-columns of many fjords further contribute to the preservation of OC (Howe et al., 2010; Paetzel and Schrader, 1992). These attributes make fjords prime locations to study the carbon cycle but also for high-resolution palaeoreconstructions of terrestrial and marine climate change.

The deposited OC is made up of terrestrial organic carbon (OC_terr) and marine organic carbon (OC_mar); however, there is a general lack of data showing the relative abundance of OC_terr and OC_mar in fjord basins (Skei, 1983). Researchers have only recently started to elucidate the relative contributions of OC_terr and OC_mar to fjordic sediments (Nuwer and Keil, 2005; Smittenberg et al., 2004; Walsh et al., 2008). Variations in the relative abundance of carbon from different carbon pools can offer insights into variable climatic conditions in response to, for example, river discharge (Weijers et al., 2009) and erosion driven by catchment precipitation (Smittenberg et al., 2004), or changes in oceanography and sea level (Knies, 2005).

Beyond studying climate change, the relative contributions of different carbon pools to fjordic sediments also offer the opportunity to model changes in marine primary productivity through geological time (Knies, 2005; Knies and Mann, 2002). Global satellite observations and microcosm experiments show that changing climate affects marine primary productivity (Behrenfeld et al., 2006; Gao et al., 2012). Since primarily produced organic matter (OM) forms the bases of the marine food chain and carbon (marine and terrestrial) sequestered in marginal basins can act as a sink for CO₂ (Borges et al., 2005), it is imperative to better understand climatic controls on palaeoproductivity (PP) over geological timescales.

Here, we show the changing OC_terr and OC_mar contributions to the sediment of Ísafjarðardjúp Fjord. Furthermore, we use the sedimentary OC_mar content to model changes in Holocene PP. Recent studies have highlighted the need for a multiproxy approach to elucidate changing OC_terr contributions to fjordic sediments (Belicka and Harvey, 2009; Huguet et al., 2007; Walsh et al., 2008). Therefore, we employ the n-alkane/alkenone-index (Marret et al., 2001), the branched versus isoprenoid tetraether (BIT)-index (Hopmans et al., 2004) and the C/N ratio (Meyers, 1997) in three binary mixing models to reconstruct OC_terr contributions throughout the Holocene. The alkenones are derived from haptophyte algae and constitute the marine component of the n-alkane/alkenone-index (Volkman et al., 1995, 1998), while the long-chained, odd-carbon numbered n-alkanes are derived from terrestrial higher plants (Eglinton et al., 1962). The BIT-index compares the relative amounts of predominantly terrestrial-derived branched glycerol-dialkyl-glycerol-tetraether (GDGTs) with the relative amount of crenarchaeol produced by marine Thaumarchaeota (Hopmans et al., 2004). Finally, terrestrial and marine OM has distinct C/N ratios allowing for the differentiation between these two sources (Lamb et al., 2006).

Methods

Site description and sampling

The 3890 cm piston core (MD99-2266) was retrieved on Leg III of the 1999 IMAGES V cruise aboard the R/V Marion Dusfresne from 106 m water depth from Ísafjarðardjúp fjord in Northwest Iceland (66° 13′77″ N, 23° 15′93″ W; Figure 1; Quillmann et al., 2010, and references therein).

Figure 1.

Core location and main surface water masses in the North Atlantic (modified after Hansen and Østerhus, 2000; source of North Atlantic map: Schlitzer, 2010). Arrows indicate warm, saline MNAW, the NAW and the cold PWs. The CSC and the NWAC originate from the NAW. The NAC, originating from the MNAW branches into the FC and the IC. Part of the IC flows through the Denmark Strait and forms the NIIC on the North Icelandic Shelf, while another part turns southwards. The EIC originates from the PW. The EIC, a branch of the PW, splits off north of Iceland and meets the NIIC. Inset: Vestfirdir Peninsula and Denmark Strait Bathymetry. The MD99-2266 core location is marked with a black dot in the mouth of Ísafjarðardjúp Fjord (modified after Quillmann et al., 2010).

The age model published by Quillmann et al. (2010) is used to calculate sediment accumulation rates (Figure 2), assuming linear sedimentary deposition between each ¹⁴C-accelerator mass spectrometry (AMS)-dated horizon. Sample ages are given in calibrated (kilo)ages before present (cal (k)a BP; present: ad 1950; Stuiver et al., 1998). The age of each sample was calculated using the sediment accumulation rate determined by the ¹⁴C-AMS dates bracketing the section of sediment from which the sample was taken. The ages of seven samples below the youngest ¹⁴C-AMS dated sediment horizon were extrapolated, assuming that the sedimentation rate between the core top and the youngest ¹⁴C-AMS date is the same as that of the sediment interval bracketed by the two youngest ¹⁴C-AMS dates.

Figure 2.

Age model of core MD99-2266. It is based on 19 ¹⁴C-AMS-dated sediment horizons and the depth horizon of the Saksunarvatn tephra (dashed line), which is dated at 10,180 ± 120 cal. a BP (Gronvold et al., 1995; Quillmann et al., 2010). Sedimentation rates are calculated using the calibrated ages of the dated horizons.

Elemental analysis

The total organic carbon (TOC) and total organic nitrogen contents of 156 samples were analysed following the method of Verardo et al. (1990). A range of 0.1–1.0 mg acetanilide standards were analysed to correct instrumental drift, and all standards and samples were blank corrected. The mean standard deviation from 17 triplicate analyses was ±3.87% for nitrogen and ± 2.47% for carbon. The mean standard deviation of the molar C/N ratio is ±0.14.

The C/N ratio shows changes in the nitrogen concentration, while the N/C ratio reflects the amount of OC (Perdue and Koprivnjak, 2007). Since the C/N ratio is more commonly used in the scientific literature, we follow Weijers et al. (2009) in giving the C/N ratio in the text, but using the N/C ratio to model the sedimentary OC_terr contribution.

Sample preparation

Freeze-dried and powdered samples were extracted by ultrasonication using a dichloromethane/methanol mixture (3:1 v/v). An internal standard consisting of squalane, 2-nonadecanone, 1-nonadecanol and erucic acid was added to each sample. An aliquot of each sample was separated into four fractions using silica gel column chromatography after Bendle et al. (2007). Aliphatic and alicyclic hydrocarbons were eluted using 4 mL of n-hexane. Aromatic hydrocarbons were eluted using 2 mL of n-hexane/dichloromethane (2:1 v/v). Ketones, n-alcohols and aldehydes were eluted with 4 mL of dichloromethane, and acids, sterols, tetraether-lipids and diether-lipids were eluted using 5 mL of methanol/dichloromethane (95:5 v/v). The tetraether containing fraction of each sample was re-dissolved in 200 µL of n-hexane/i-propanol (99:1 v/v) and filtered using a 0.45 µm polytetrafluoroethylene (PTFE) syringe filter prior to analysis.

Biomarker analysis

The alkenones and n-alkanes in 326 samples were analysed using a gas chromatograph (GC; Shimadzu 2010) with a flame ionisation detector (FID). The carrier gas was hydrogen (constant pressure; 190 kPa). The separation of the different compounds was achieved using one of two identical columns, either a BP1 (SGE Analytical Science) or a TG-1MS (Thermo Scientific) column (length: 60 m, diameter: 0.25 mm, film thickness: 0.25 µm, coating: 100% dimethyl-polysiloxane). The GC oven was held at 60°C for 2 min, then the temperature was ramped up to 120°C at 30°C/min and then to 350°C at 3°C/min, where the temperature was held for 20 min. An injection standard consisting of methyl behenate was added to each sample prior to analysis.

The alkenones and n-alkanes were identified by comparing the retention time of the substances in the samples to the retention time of standard substances and by using a Shimadzu OP2010-Plus Mass Spectrometer (MS) interfaced with a Shimadzu 2010 GC. The carrier gas was helium (constant pressure: 230 kPa). The ion source and interface temperatures were 200°C and 300°C, respectively. The GC columns and temperature programme were identical to the GC-FID analysis. The ionisation energy was 70 eV, and the scan width was 50–800 mass units.

The n-alkanes and alkenones were quantified by comparing their peak areas with the peak areas of the internal standard. The mean analytical quantification errors (mean standard deviation) of 10 n-alkane and 34 alkenone-containing fractions run in duplicate were 5.5% and 3.3%, respectively.

The n-alkane-/alkenone-index proposed by (Marret et al., 2001) and modified by Weijers et al. (2009) was calculated using the concentrations of the n-alkanes and alkenones (Figure 3).

n - alkane/alkenone-index = \frac{odd-chained (C_{25} - C_{35}) n -alkanes}{[odd-chained (C_{25} - C_{35}) n -alkanes] + 3 (C_{37} alkenones)}

(1)

Figure 3.

Biomarker and bulk parameter variability of core MD99-2266. (a) TOC content, black triangles indicate the ¹⁴C-AMS-dated sediment horizons, (b) C/N ratio, (c) BIT-index, (d) odd-chained (C₂₅–C₃₅) n-alkane/alkenone index, (e) concentration of odd-chained (C₂₅–C₃₅) n-alkanes and (f) concentration of C_37:2 and C_37:3-alkenones. Grey dots and analytical error bars signify values of individual samples and the black line indicates the moving average.

The relative tetraether abundances in 299 MD99-2266 sediment samples were analysed using high-performance liquid chromatography–atmospheric pressure chemical ionisation–mass spectrometry (HPLC-APCI-MS) at the Organic Geochemistry Unit at the University of Bristol. The analyses were conducted on a Thermo Scientific TSQ Quantum Access equipped with an Acella Autosampler, Acella pump and Xcalibur software. The LC was equipped with an Alltech Prevail Cyano column (150 mm × 2.1 mm; film thickness: 3 µm). Two mobile phases, n-hexane (A) and i-propanol (B) were used at a flow rate of 0.2 mL/min. Initially, 1% of B v/v was held for 7 min. Then the concentration of B was increased on a linear gradient over 43 min to 1.6%. The concentration of B was increased to 10% v/v at 51 min and held for 2 min. Finally, the concentration of B was decreased to 1% v/v and held for 10 min. Single ion mode (SIM) was used to monitor the abundance of the [M+H]⁺ (molecular ion + proton) ion. The peak areas were used to calculate the relative abundance of each GDGT. The BIT-index was calculated using the equation published by Hopmans et al. (2004; Figure 3c). Roman numerals indicate relative abundances and structures of GDGTs as described by Hopmans et al. (2004; see also supplementary material S3, Figure S2).

BIT = (I + II + III) / [(I + II + III) + (IV)]

(2)

The mean standard deviation of BIT-index values is ±0.01 determined by the analyses of nine samples in triplicate and two in duplicate.

Statistical analyses

Statistical analyses were performed using SigmaPlot 11.0 (Systat Software, Inc.). Data were smoothed using the Arand time series software (Howell et al., 2006). The analytical error (standard deviation) of the measured variables has been propagated through the equations where appropriate (see supplementary material S1).

Mass accumulation rates

Mass accumulation rates (MARs) for TOC (mg cm/a) and individual biomarkers (ng cm/a) were calculated using Eq. (3) (Rommerskirchen et al., 2003, and references therein), where X is the TOC or specific biomarker concentration in milligrams or nanograms per gram of dry sediment (mg/g Sed or ng/g Sed), respectively, ρ is dry bulk density (DBD; g/cm³) and LSR is the linear sedimentation rate (LSR; cm/a; Figure 4). The DBD was obtained from the project collaborator Ursula Quillmann.

Figure 4.

Sedimentation and MARs of core MD99-2266. (a) Linear sedimentation rate of core MD99-2266, black triangles indicate the ¹⁴C-AMS dated sediment horizons; (b) MAR of odd-chained (C₂₅–C₃₅) n-alkanes; (c) MAR of TOC; (d) MAR of the sum of C_37:2- and C_37:3-alkenones.

MAR = X * ρ * LSR

(3)

Modelling OC_terr contributions

The molar C/N ratio, n-alkane/alkenone-index and the BIT-index were employed in a binary mixing model (Eq. (4)) previously used by Weijers et al. (2009) to assess the percentile contribution of OC_terr (f_terr) to the TOC pool of Ísafjarðardjúp Fjord sediment (Figure 5).

f_{terr} = \frac{X_{sample} - X_{Mar}}{X_{Terr} - X_{Mar}} \cdot 100 %

(4)

X_sample, X_Mar and X_Terr are the values of the sample, the marine end member value and the terrestrial end member value of the C/N ratio, the n-alkane/alkenone-index and the BIT-index, respectively.

Figure 5.

Estimates of the OC_terr contribution to the sedimentary TOC pool using a binary mixing model approach. (a) OC_terr estimate using the n-alkane/alkenone index, black triangles indicate the ¹⁴C-dated sediment horizons; (b) OC_terr estimate using the BIT-index; (c) the C/N ratios, grey dots with propagated standard deviations represent individual sample values, the black line indicates the moving average and (d) mean OC_terr estimate (black line) and standard deviation of the mean (grey-shaded area). Only samples where the n-alkane/alkenone index, BIT-index and C/N ratio data were available were used to compile the mean OC_terr estimate (n = 137).

Marine sourced OC and modelling marine paleoproductivity

The sedimentary TOC comprises two carbon pools, OC_terr and OC_mar. The OC_mar content of the fjordic sediment is calculated using Eq. (5).

O C_{mar} = TOC - O C_{terr}

(5)

The marine PP (gC·m²/a) was calculated using the model published by Knies and Mann (2002; Eq. (6)), where OC_mar is the marine OC content (%), DBD is dry bulk density (g/cm³), LSR is the linear sedimentation rate (cm/ka) and D is the water-column depth (m).

PP = {\frac{O C_{mar} \cdot 0.378 \cdot DBD \cdot LSR \cdot D^{0.63}}{[1 - (\frac{1}{0.037 \cdot LS R^{1.5} + 1})]}}^{0.71}

(6)

Early Holocene sea-level change in the Ísafjarðardjúp Fjord as a response to final deglaciation and the isostatic rebound of Iceland is considered in the PP estimates. Quillmann et al. (2010) show that the relative sea level decreased by ~30 m between ~10,700 and ~8900 cal. a BP. Subsequently, relative sea level increased again and reached its contemporary level at ~5700 cal. a BP. Assuming a linear sea level decrease and subsequent increase, the relative sea-level change was calculated using today’s sea level as a reference point.

Results

Bulk parameters

The LSR varies from 0.09 to 4.09 cm/a (Figures 2 and 4a). The highest LSR coincides with the Saksunarvatn tephra at 10,180 ± 120 cal. a BP (Gronvold et al., 1995; Quillmann et al., 2010). A second peak in sedimentation rate at ~8250 cal. a BP is likely an artefact caused by the close proximity of the adjacent ¹⁴C-AMS dates, as there is no visual disturbance in sedimentation in that interval (Quillmann, personal communication, 2013). The sedimentation rates are highest in the early Holocene and decrease towards the late Holocene.

The TOC content of the sediment core varies between 0.7% and 2.2% throughout the Holocene (Figure 3a). The mean TOC content is 1.2%. Over the first ~2400 years, the TOC content increases before decreasing from ~8400 to ~8100 cal. a BP. The TOC content fluctuates between 1% and 1.5% from ~8000 and ~3000 cal. a BP with the exception of a TOC spike at ~4600 cal. a BP. Throughout the last ~2000 years of the record, TOC values increase.

The MAR of TOC follows the sedimentation rate closely (Figure 4b). The highest values are recorded during the early Holocene with the highest value of 34 mg/cm²/a at ~10,100 cal. a BP.

The molar C/N ratio increases throughout the first ~7800 years of the record from values of 3.6 to values of nearly 7 (Figure 3b). The variability of the C/N ratio increases throughout the last ~3500 years of the record compared with the previous ~7000 years, and the clear tendency towards increasing C/N values is not observed. Excursions to C/N values of >6.5 occur at ~3300 and from ~2700 to ~2300, and at ~820 cal. a BP.

Biomarkers

Throughout the record, the BIT-index does not rise above 0.15, and the lowest values are seen in the early Holocene, with a mean value of 0.04 between ~10,800 and ~8300 cal. a BP (Figure 3c). BIT-index values increase from ~8300 cal. a BP, and the highest values of 0.13 and 0.14 are reached between ~4000 and ~3000 cal. a BP.

A total of 16 n-alkane-containing fractions were discarded due to contamination (see supplementary material S2; Figure S1). The average concentration of the odd-chained (C₂₅–C₃₅) n-alkanes is 537 ng/g Sed (Figure 3e). The n-alkane concentration varies between 200 and 800 ng/g Sed throughout most of the Holocene (~10,800 to ~1200 cal. a BP) with one exception at ~6900 cal. a BP, where the concentration increases to just over 1000 ng/g Sed, and three exceptions between ~7600 and ~8100 cal. a BP, where the concentration falls to ~150 ng/g Sed. The odd-chained n-alkane concentrations rise after ~1200 cal. a BP to their highest values of nearly 1400 ng/g Sed.

The MAR of the odd-chained n-alkanes follows the sedimentation rate (Figure 4b). The highest value of just over 1000 ng/cm²/a is recorded at ~10,100 corresponding to the high LSR.

The mean combined concentration of the C_37:2- and C_37:3-alkenones is 1134 ng/g Sed throughout the Holocene (Figure 3f). The highest concentration of alkenones of 3000 ng/g Sed is recorded at ~9000 cal. a BP, and the lowest concentration of 120 ng/g Sed is recorded at 8200 cal. a BP.

The MAR of the sum of the C_37:3 and C_37:2 alkenones follows variations of LSRs (Figure 4d). During the early Holocene, from ~10,800 to ~7000 cal. a BP, the alkenone MAR exhibits the highest variability. Alkenone MARs of 2210 and 1600 ng/cm²/a are the highest, recorded at ~10,100 and ~9000 cal. a BP, respectively.

High n-alkane-/alkenone-index values indicate increased OC_terr input, while lower values suggest a higher proportion of OC_mar input (Figure 3d). The average value of the index throughout the Holocene is 0.15. The highest fluctuation in the index is seen between ~10,800 and ~8000 cal. a BP, owing to the high variability of the C₃₇-alkenone concentration during that interval. Throughout the rest of the Holocene, the n-alkane/alkenone index values are below 0.3.

Modelling the terrestrial OC contribution to the sediments of Ísafjarðardjúp Fjord

The percentage of terrestrial OC contributing to the sedimentary TOC pool is estimated using three different proxies in a binary mixing model (Figure 5). The choice of terrestrial and marine end member values is discussed below.

The mean sedimentary OC_terr contribution estimated by the n-alkane/alkenone index is 14%. The OC_terr contribution estimated by this proxy is highest in the oldest part of the record, from ~10,800 to ~8000 cal. a BP. This proxy is the only one showing OC_terr estimates higher than 30% in the early Holocene, between ~9000 and ~8000 cal. a BP.

The mean estimated OC_terr contribution to the fjordic sediment is 8% when using the BIT-index in the binary mixing model. The OC_terr contributions remain low during the early Holocene until ~8500 cal. a BP, at which point the OC_terr estimates sharply increase. The OC_terr contributions continually increase until ~3000 cal. a BP, before slowly decreasing again.

The C/N ratio–inferred OC_terr estimates show the highest variability. The mean estimated OC_terr contribution to the sediment is 23%. In the early Holocene, some of the estimated OC_terr values are negative. From ~8000 to ~4000 cal. a BP, the C/N ratio estimated OC_terr increases from below 10% to above 40%. In the latter part of the record, the estimated OC_terr remains high.

Sedimentary OC_mar content and modelled paleoproductivity

The mean estimated OC_terr values derived from the three binary mixing models were used to calculate the OC_mar content of MD99-2266. The concentrations and MARs of TOC, OC_terr and OC_mar are shown in Figure 6. Throughout the Holocene, the TOC MAR correlates well with OC_mar (Spearman rank order correlation coefficient: 0.99; p < 0.05), but less so with OC_terr (Spearman rank order correlation coefficient 0.76; p < 0.05).

Figure 6.

Concentration and MARs of total, marine and terrestrial OC. The error bars indicate the propagated analytical error (mean standard deviation) of the mean OC_terr estimates and the analytical error of the TOC measurements. Only those samples where the mean OC_terr estimate was calculated were used to estimate OC_mar (n = 137).

The marine PP of Ísafjarðardjúp Fjord was estimated using the estimated OC_mar content and the model by Knies and Mann (2002; Figure 7c). The model estimates primary productivity values of around 300 gC/m²/a between ~10,000 and ~8300 cal. a BP and a PP peak of 500 gC/m²/a at ~8200 cal. a BP followed by a sharp decrease. After 8000 cal. a BP, PP values gradually decline and reach their minimum between ~2300 and ~2500 cal. a BP. Throughout the youngest part of the record, PP values rise again sharply and vary around ~200 gC/m²/a.

Figure 7.

Holocene palaeoclimate records compared with the OC_terr and palaeoproductivity variability of Ísafjarðardjúp Fjord. (a) Summer insolation at 60°N (Laskar et al., 2004). The black triangles indicate the ¹⁴C-Α;ΜS-dated sediment horizons of core MD99-2266. (b) Mean OC_terr estimate (black line) and standard deviation of the mean (grey-shaded area). (c) Marine palaeoproductivity of Ísafjarðardjúp Fjord modelled after Knies and Mann (2002). The error bars indicate the propagated analytical error of the percentile OCmar concentration. Only those samples where the mean OC_terr estimate was calculated were used to estimate marine palaeoproductivity (n = 137). (d) Reconstructed NAO (NAOms) variability (Trouet et al., 2009). (e) Variations in the speed of the ISOW south of Iceland linked to the speed of the meridional overturning circulation (50 year running average; Hall et al., 2004). (f) August sea surface temperatures based on diatom assemblages (50 year running average; Justwan et al., 2008). (g) GISP 2 oxygen isotope inferred temperature variations of the North Atlantic sector (Grootes and Stuiver, 1997). The MCA, the neoglaciation and the Holocene thermal maximum are indicated by dashed lines, the 8.2 event is indicated through a grey bar.

Discussion

Factors influencing estimates of OC_terr contribution to the fjordic sediments

The estimated percentile terrestrial OC fraction of the TOC depends on the end member values employed in the binary mixing model. For the n-alkane/alkenone index, X_Terr is 1 and X_Mar is 0, assuming that the n-alkanes are produced by terrestrial higher plants and alkenones are produced in situ by marine haptophytes only, and that there is no, or negligible, input of alkenones from lakes (Castañeda and Schouten, 2011). Furthermore, it is assumed that n-alkanes and alkenones are representative fractions of the OC_terr and OC_mar contributions into the sediment, and that their concentrations vary in step with changes of OC_terr and OC_mar contributions.

The X_Terr value used to evaluate the terrestrial OC input via the BIT-index is 0.91 and not 1, as crenarchaeol is found in soils and in marine environments (Weijers et al., 2006, 2009). Assuming that branched GDGTs are produced by terrestrial bacteria, 0 is used as the marine end member value. However, branched GDGTs likely have an aquatic source as well (Fietz et al., 2011, 2012), therefore the marine end member value is likely higher than 0. Furthermore, Fietz et al. (2012) have shown that the concentration of crenarchaeol in marine sediments co-varies with the concentration of the branched GDGTs used in the BIT-index. The linear regression (r² = 0.76; supplementary material S3, Figure S3) between the relative abundance of crenarchaeol and the combined relative abundances of the branched GDGTs confirms the co-variance in the sediment core studied here. Fietz et al. (2012) argue that Thaumarchaeota may be more productive during periods where more terrestrial matter is washed into the water-column, as indicated by high amounts of branched GDGTs. However, the increase of the BIT-index throughout the Holocene broadly agrees with the other two proxies, suggesting that the BIT-index does reflect OC_terr inputs.

The terrestrial organisms producing the branched GDGTs used to calculate the BIT-index are specific to soil and peat environments (Hopmans et al., 2004; Weijers et al., 2006). Thus, results of the binary mixing model employing the BIT-index likely underestimate the amount of terrestrially derived OC contributing to the TOC pool by exclusion of fresh higher plant material (as represented by the n-alkane plant waxes).

Different OM pools have widely varying C/N ratios (Lamb et al., 2006), with typical C/N values for algae between 4 and 10, while vascular land plants have C/N values of ≥20 (Meyers, 1994). In order to be sure, that the marine end member of the C/N ratio exclusively represents a marine source, 4 is used as the representative value of X_Mar, assuming that algae are the sole or at least main contributor of marine sourced OC to the sediment.

Possible X_Terr end member values for use in the binary mixing model employing the C/N ratio exhibit considerable variability in Northwest Iceland (Table 1). Most soils in Iceland have C/N ratios between 11 and 17 (Pitty, 1979). Due to the large variety of C/N values from different terrestrial carbon sources, the choice of a ‘true’ C/N end member value, representing 100% of terrestrial OC input is difficult. Here, we choose the C/N value of the sedge Carex cf. rariflora (C/N = 48) that dominates the wetland environment north of Ísafjarðardjúp Fjord today (Skrzypek et al., 2008). Pollen analysis show that the vegetation on Vestfirdir Peninsula was made up of sedge and shrub type plants from ~10,100 cal. a BP and that sedge-dominated wetlands were a significant and highly resilient part of the landscape (Caseldine et al., 2003), supporting the choice of the X_Terr value. If a C/N value typical for Icelandic soils (C/N = 15) is chosen as the terrestrial end member value, then f_Terr increases by 20%. Therefore, the choice of end-members for the binary mixing model has a profound effect on the result, indicating that they need to be interpreted with caution.

Table 1.

C/N values of different samples from terrestrial sources in northwest Iceland. The C/N value of the sedge Carex cf. rariflora (*) is used as the terrestrial end member value when converting C/N values into percentile OC_terr values.

Sample type	No. of samples	Location	C/N ratio	Literature
Terrestrial plants	6	64°32′ N; 22°31′ W	41.3–72.6	Langdon et al. (2010)
Moss Warnstorfia exannulata	1	66°06′ N; 22°23′ W	62	Skrzypek et al. (2008)
Segde C. cf. rariflora*	1	66°06’ N; 22°23′ W	48*	Skrzypek et al. (2008)
Peat (upper 10 cm)	1	64°32’ N; 22°31′ W	15.4	Langdon et al. (2010)
Soil	3	64°32’ N; 22°31′ W	16.5–22	Langdon et al. (2010)
Carex peat	5	66°06′ N; 22°23′ W	33–44	Skrzypek et al. (2008)

OC_terr contribution to the fjordic sediments

The n-alkane/alkenone index indicates the highest OC_terr contribution between ~8000 and ~9000 cal. a BP (Figure 5a). During that period, concentrations of C₃₇-alkenones in the sediment are among the lowest of the Holocene, while n-alkane concentrations range around the mean concentrations during Holocene. Thus, the estimated high OC_terr contribution between ~8000 and ~9000 cal. a BP is not caused by an actual high terrigenous input into sediments, but rather by relatively low C₃₇-alkenone concentrations, suggesting adverse growing conditions for alkenone-producing haptophytes. This explanation is supported by reduced salinity of Ísafjarðardjúp Fjord waters caused by meltwater events that had a strong influence on the fjordic environment before ~8000 cal. a BP (Quillmann et al., 2010, 2012). The influx of meltwater may have impaired the growth of the resident alkenone-producing algae.

The estimated OC_terr contribution between ~8000 and ~1200 cal. a BP fluctuates between ~10% and ~20%. During the last ~1000 years of the record, the concentration of the n-alkanes increases, and this increase of terrestrially derived matter is reflected in the rising n-alkane/alkenone-index OC_terr contribution reaching 30% at ~300 cal. a BP.

The two periods of relatively high estimated OC_terr values due to low alkenone concentration in the early Holocene and high n-alkane concentrations in the late Holocene highlight the necessity to cautiously approach the sedimentary OC_terr estimates. Furthermore, the n-alkane/alkenone-index may underestimate the OC_mar contribution, if the assumption is made that alkenones are preferentially degraded compared to n-alkanes (Hoefs et al., 2002; Sinninghe Damsté et al., 2002).

Compared to the other two proxies, the BIT-index provides the lowest percentile OC_terr estimates throughout the record. This is expected, as the BIT-index primarily reflects changing contributions of soil OM and excludes higher plant OM (Hopmans et al., 2004; Walsh et al., 2008; Weijers et al., 2006). The observed correlation between the concentrations of the crenarchaeol and the branched GDGTs also suppresses the BIT-index and suggests a potential contribution of aquatic branched GDGTs (e.g. Fietz et al., 2012).

OC_terr estimates show the highest variability when using C/N ratios. The C/N values of the samples which produce negative OC_terr values are lower than the X_Mar end member value, explaining the negative OC_terr estimates in the early Holocene. Lamb et al. (2006) show that bacterial C/N values can be lower than 4. Therefore, very low C/N values due to the contribution of bacterial OM to the total OM pool could, in conjunction with very low terrestrially derived organic matter, cause very low C/N values.

The OC_terr increase between ~8000 and ~4000 cal. a BP, based on changing C/N ratios, may indicate a deteriorating climate. In a lacustrine environment, C/N values have been shown to increase with decreasing temperatures (Axford et al., 2009). Axford et al. (2009) attribute the changing C/N values to either changes in primary production or changes in the flux of terrestrial material into the lacustrine sediments. Thus, increasing C/N values throughout the Holocene are associated with deteriorating climatic conditions, as discussed in the following section.

Variations of OC_terr in response to environmental and/or anthropogenic influences

The three model approaches used to estimate the amount of OC_terr show divergent results highlighting the variability of the proxies applied to the problem of quantifying sedimentary OC_terr input. By combining the OC_terr estimates by the three different proxies into one record showing the mean OC_terr content of the fjordic sediment, the most plausible approximation of broad Holocene OC_terr trends is derived (Figures 5d and 7b). However, this results in an inevitable loss of finer detail that is observed in individual records (see supplementary material S4).

The combined record of OC_terr estimates shows two periods where the OC_terr content in the sediment rises. Throughout the early Holocene, from ~10,700 to ~8800 cal. a BP, the OC_terr content in the sediment increases from 2% to 18%. The early Holocene was characterised by high Northern Hemisphere summer insolation, and glaciers on Iceland were continually retreating (Geirsdottir et al., 2009), freeing up land area for soil formation and subsequent increase in vegetation cover. This interpretation is supported by increasing pollen concentration in lacustrine sediments on Vestfirdir Peninsula and North Iceland (Caseldine et al., 2003; Langdon et al., 2010) and explains the OC_terr increase.

From ~8600 to ~8000 cal. a BP, the contribution of OC_terr tends to decrease (Figure 7b). Several marine archives from the Denmark Strait and the North Icelandic Shelf have recorded either one cooling event centred around 8200 cal. a BP (Giraudeau et al., 2000; Knudsen et al., 2004; Ólafsdóttir et al., 2010; Quillmann et al., 2012), or a number of shorter cooling events (Jennings et al., 2011). Based on chironomid assemblages, temperatures in Northwest Iceland decreased slightly at ~ 8200 (Caseldine et al., 2003) and at ~8500 cal. a BP (Langdon et al., 2010). This downturn of climate could have caused a reversal or halting in the development of soil reservoirs and vegetation, thus decreasing the amount of OC_terr contribution to the sediment. Indeed, Hallsdóttir (1995) notes that the development towards subalpine birch woodland suddenly halted at ~7500 ¹⁴C years (~8300 cal a BP). Ólafsdóttir et al. (2001) have modelled a dramatic decrease in vegetation cover at ~8000 cal a BP. The sharp increase in soil OM contribution to the sediments as indicated by the BIT-index (Figure 3) at 8.2 cal. a BP supports this interpretation. Decreased vegetation cover, related to the 8.2 event (Figure 7; Alley and Ágústsdóttir, 2005), would have led to more soil erosion and thus to higher input of soil OC into the sediments.

Despite the short, cold interval causing a decrease of the OC_terr contribution as previously discussed, the broader period from ~8600 to ~5400 cal. a BP was regarded as the warmest episode of the Holocene (Figure 7; Kaufman et al., 2004; Knudsen et al., 2008). This warm climate was conducive to extensive soil and vegetation development in Iceland (Caseldine et al., 2003; Hallsdóttir, 1995; Wastl et al., 2001) and likely explains both the initial OC_terr increase in the fjordic sediments after ~8000 cal. a BP, during a recovery phase, and then a period of relative stability from ~7500 to ~5000 cal. a BP as carbon was stored up in soil reservoirs.

The marked increase of OC_terr input to the sediments after ~5000 cal. a BP is attributed to the climatic deterioration following the Holocene Thermal Maximum that is apparent in marine as well as terrestrial records (Figure 7f; Jennings et al., 2002; Justwan et al., 2008; Principato, 2008; Wastl et al., 2001). The deteriorating climate possibly caused accelerated soil degradation due to periodic freezing and thawing making soils and vegetation cover more susceptible to erosion by high velocity winds (Jackson et al., 2005; Ólafsdóttir and Gudmundsson, 2002). Furthermore, birch woodland was in retreat throughout the neoglaciation shown by decreasing Betula sp. pollen concentrations (Hallsdóttir, 1995; Hallsdóttir et al., 2005). Heath land and mires expanded, and in the lowlands, woods were replaced and buried by peat (Hallsdóttir et al., 2005). Changing soil and vegetation types as well as increased amounts of peat could have caused increased amounts of OC_terr to be transported into the fjord, compounding the effects of increased soil erosion as discussed above.

The sharp increase in the estimated contribution of OC_terr to the fjordic sediments between ~1200 and ~600 cal. a BP may have been caused by anthropogenic actions and/or climatic fluctuations. The period from ~1100 to ~550 cal. a BP, known as the Medieval Climate Anomaly (MCA; Hughes and Diaz, 1994), was characterised by warm climate throughout Europe (Buntgen et al., 2011; Graham et al., 2011). The Norse established settlements in Iceland at ~ad 870 (~1080 cal. a BP; Andrews et al., 2001). The first generation of settlers cleared woodland to make space for farmland (Hallsdóttir et al., 2005) likely causing increased soil erosion (Dugmore et al., 2000). Human-driven vegetation change is probably reflected by a drastic rise in the odd-chained n-alkane concentration at ~1200 cal. a BP (Figure 3). Increased soil erosion, along with possible burning of woodland to make space for agricultural land, could therefore explain increased amounts of OC_terr in the sediments.

A second explanation is provided by the change in climate itself. The ameliorated climate of the MCA has been associated with a prevailing positive North Atlantic Oscillation (NAO) phase (Figure 7d; Trouet et al., 2009) that is associated with increased precipitation in Iceland (Hurrell, 1995). Increased precipitation would have caused more terrestrial runoff resulting in a higher amount of OC_terr to be washed into the fjord.

Marine paleoproductivity

Today’s average annual primary production around Vestfirdir Peninsula lies between 200 and 400 gC/m²/a (Astthorsson et al., 2007; Longhurst et al., 1995; Zhai et al., 2012). The Knies and Mann (2002) model produces average PP values of 170 gC/m²/a in the most recent 100 years of the record, suggesting that it may underestimate PP. If the sedimentary C₃₇-alkenone concentrations are used in the model instead of the sedimentary OC_mar content, the PP values are on average 80% lower throughout the Holocene (see supplementary material S5 and Figure S4).

The primary production in Ísafjarðardjúp Fjord may have reached 350 gC/m²/a between ~10,000 and ~8000 cal. a BP as indicated by the model (Figure 7c). Similar high primary production values are found in the Arctic today in areas with a high supply of nutrients due to upwelling or riverine input (Wollenburg et al., 2004, and references therein). The apparent peak in primary production at ~8250 cal. a BP is likely an artefact, as it coincides with a sedimentation rate anomaly and is not reflected in the alkenone data (Figure 3). However, we suggest the boarder period of high productivity between ~10,000 and ~8000 cal. a BP is linked to the amount of nutrients being washed into the fjord. Quillmann et al. (2010) show that the benthic foraminifera species Fursenkoina fusiformis, that is associated with high nutrient inputs, was most abundant during the early Holocene while sedimentation rates were high. Hence, the relatively high PP in the early Holocene was likely driven by high amounts of terrigenous material being washed into the fjord as shown by the close correlation between OC_mar and the sedimentation rate (Figure 6).

Besides the terrestrial supply of nutrients from the catchment, an allochthonous supply of advected marine nutrients may also have played a major role in affecting PP throughout the Holocene. A maximum in primary production from 8000 to 6000 cal. a BP on the northwest Norwegian coast has been attributed to a strong inflow of nutrient-rich Atlantic water (Knies, 2005), and periods of high primary productivity in the Fram Strait have also been associated with an increased nutrient supply (Wollenburg et al., 2004). The primary production in Ísafjarðardjúp Fjord rises dramatically from below 200 to 300 gC/m²/a at ~10,200 cal. a BP coincidental with the Irminger Current, which supplies Atlantic water to the Denmark Strait fully penetrating the area (Ólafsdóttir et al., 2010). After ~8000 cal. a BP, the primary production decreases and reaches values varying around 100 gC/m²/a at ~3000 cal. a BP, coinciding with a diminishing influence of the Irminger Current in the waters surrounding Northwest Iceland (Giraudeau et al., 2004; Koc et al., 1993; Ólafsdóttir et al., 2010). Today, periods with increased influx of Atlantic water show significantly increased primary production on the Northwest Icelandic Shelf (Jónsson and Valdimarsson, 2012, and references therein). This is compelling evidence that the long-term changes in primary production in Ísafjarðardjúp Fjord have been forced by variations in the nutrient supply brought about by either Arctic or Atlantic water masses dominating the area. The influence of Atlantic water masses in the Denmark Strait have been controlled by changing climate drivers such as changes in Northern Hemisphere insolation (Koc and Jansen, 1994), and changes in the strength of the meridional overturning circulation (Figure 7e; Bianchi and McCave, 1999; Hall et al., 2004; Hoogakker et al., 2011) but also by variations in atmospheric circulation patterns in the Denmark Strait (Blindheim and Malmberg, 2005). Therefore, changes in the amount of primary production throughout the Holocene may have at least indirectly been forced by climatic change.

Conclusion

In this study, the contributions of OC derived from terrestrial and marine sources to Ísafjarðardjúp Fjord sediments have been estimated. Three biomarker and bulk proxy data sets in binary mixing models were used to calculate the contribution of OC_terr to the TOC pool. Subsequently, the results of the three models were combined to produce the most conservative and accurate OC_terr estimate possible.

The estimated OC_terr contribution to the sediment increases throughout the Holocene to a maximum of 25%. The OC_terr influx into the sediments is not strongly correlated with sedimentation rate but rather controlled by climatic changes. The multiproxy approach to estimating OC_terr contribution highlights the large uncertainties that are still associated with the different biomarkers and bulk approaches. Hence, further studies are needed to model OC_terr contributions to marine sediments more accurately.

Changes in Holocene PP were estimated using the model published by Knies and Mann (2002) and the reconstructed OC_mar contribution to the fjordic sediments. It appears that changes in the PP throughout the Holocene were forced by changing amounts of terrestrial catchment and advected marine nutrient supplies. Northern Hemisphere insolation change and changes in the strength of the meridional overturning circulation caused different current systems to dominate throughout the Holocene and thus drove the advected marine nutrient supply and marine primary productivity at the coring site.

Footnotes

Acknowledgements

We would like to thank Professor Philip Meyers and Dr Jochen Knies for their useful and insightful comments. This research used samples extracted from sediment core MD99-2266, collected during the 1999 IMAGES cruise programme, by CALYPSO piston coring deployed from the research vessel Marion Dusfresne II. We especially thank the crew and staff of the Marion Dufresne II, the CALYPSO coring team, Professor John Andrews and the technical staff at the Institute of Arctic and Alpine Research, Dr Richard Pancost and technical staff at the Organic Geochemistry Unit, Bristol, and the Glasgow Molecular Organic Geochemistry laboratory. We thank Dr Ellen Roosen from the Woods Hole Oceanographic Institution who sent additional u-channels of the topmost 3.5 m of the sediment core archive half.

Funding

We thank the Scottish Alliance for Geoscience, Environment and Society (SAGES) who funded the PhD to Dr Heiko Moossen.

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Supplementary Material

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Holocene changes in marine productivity and terrestrial organic carbon inputs into an Icelandic fjord: Application of molecular and bulk organic proxies

Abstract

Keywords

Introduction

Methods

Site description and sampling

Elemental analysis

Sample preparation

Biomarker analysis

Statistical analyses

Mass accumulation rates

Modelling OCterr contributions

Marine sourced OC and modelling marine paleoproductivity

Results

Bulk parameters

Biomarkers

Modelling the terrestrial OC contribution to the sediments of Ísafjarðardjúp Fjord

Sedimentary OCmar content and modelled paleoproductivity

Discussion

Factors influencing estimates of OCterr contribution to the fjordic sediments

OCterr contribution to the fjordic sediments

Variations of OCterr in response to environmental and/or anthropogenic influences

Marine paleoproductivity

Conclusion

Footnotes

Acknowledgements

Funding

References

Supplementary Material

Modelling OC_terr contributions

Sedimentary OC_mar content and modelled paleoproductivity

Factors influencing estimates of OC_terr contribution to the fjordic sediments

OC_terr contribution to the fjordic sediments

Variations of OC_terr in response to environmental and/or anthropogenic influences