Shallow lake response to Holocene climate variation in south-central Minnesota,USA

Abstract

Sedimentary deposits in lakes across the upper Midwest record the co-evolution of climate and biogeochemistry since the retreat of the Laurentide Ice Sheet at the end of the last glacial period. Here, we report on a Holocene lake sediment record from Chub Lake, a shallow (3 m depth) eutrophic lake system in south-central Minnesota. High-resolution elemental data from scanning XRF along with variations in organic matter, carbonate minerals, clastic material, biogenic silica, charcoal, and carbon isotopes reveal internally consistent patterns of hydroclimatic influence on this shallow lake system from 11,300 years BP to present. In particular, authigenic carbonate mineral formation and preservation in Chub Lake appears to be well suited as a moisture proxy beginning around 9700 BP up until ∼2300 BP, when a combination of more humid climates and basin-infilling change the hydrology of Chub Lake. This work emphasizes the importance of evaluating shallow lake sediment records both as important archives of climate proxies and case studies on how changing climates impact aquatic systems.

Keywords

Holocene paleoclimate scanning XRF shallow lake

Introduction

The long-term evolution of Holocene climate in North America is relatively well-understood, in large part due to dozens of Holocene lake sediment records from across the continent that can be compared in multiproxy ensembles. Broad trends show a climate that was cool and humid in the early Holocene, warm and arid in the Mid-Holocene, and which gradually returned to cool, humid conditions over the Late-Holocene to present (Shuman and Marsicek, 2016; Williams et al., 2009; Wanner et al., 2011). Lake sediment proxy records are useful in constraining these continental scale climate changes, but also provide insight on lake-system change and local climate response to broader patterns. For example, charcoal and pollen records have improved our understanding of fire dynamics and vegetation change along the forest-to-prairie ecotone (Camill et al., 2003, 2012; Commerford et al., 2016; Whitlock et al., 1993; Wright, 1992; Umbanhowar et al., 2009), while geochemical and magnetic properties of lake sediments help constrain important changes in both authigenic (redox state, productivity, nutrient cycling, etc.) and autogenic (wind, erosion, catchment soil development, etc.) processes (Contreras et al., 2018; Davies et al., 2015; Geiss et al., 2003; Kylander et al., 2011; McLauchlan et al., 2019; Martín-Puertas et al., 2011; Shapley et al., 2005).

However, most studies target relatively deep lakes that have the potential to record more continuous and high resolution sedimentation over time due to the limited impacts of near-shore processes and lake level change (Korponai et al., 2010; Whitmore et al., 1996). In contrast, shallow lakes (<5 m) tend to be less-well studied as they are more prone to sedimentation being interrupted by nearshore processes, changes in lake regime, changes in lake level, and drying. Yet, shallow lakes are widespread on the modern landscape and represent some of our most sensitive aquatic ecosystems (Edlund et al., 2022; Ramstack Hobbs et al., 2016).

Chub Lake is a shallow (∼3 m depth), eutrophic lake system in south-central Minnesota (Figure 1). Aerial photos confirm that Chub Lake has a history of extreme lake level fluctuation in response to hydroclimatic change (Supplemental 1). Here, we use a multi-proxy approach to characterize sedimentation within Chub Lake over the past 11,300 years. Proxy records used include elemental composition from scanning XRF, total carbon, total Nitrogen, stable isotopic composition of carbon (δ¹³C), loss-on-ignition analysis, weight percent of biogenic silica (BSi), and concentration and influx of charcoal. We evaluate these proxy records against regional climate records (Camill et al., 2003; Geiss et al., 2003) and the broader context of Holocene climate evolution (Shuman and Marsicek, 2016; Williams et al., 2009).

Figure 1.

Map of study area (upper panel) showing Chub Lake (light blue), emergent wetlands (striped areas), and Chub Creek (blue line). Location of Chub Lake is shown as red dot on inset map of Minnesota. Location of coring site in center of lake is marked with green dot. Lower panel shows a generalized cross section across the central part of the lake (dashed line on map). Subsurface geology is interpreted from nearby well-logs (labeled a–h white dots on map) compiled by the Minnesota Geologic Survey. See Supplemental Material for well-log identification numbers.

Methods

Site description

Chub lake is a shallow (∼3 m depth) glacial lake located in Dakota County, Minnesota (Figure 1). The elongated shape of Chub Lake lies within a sinuous, relatively narrow valley. The valley continues to the south of the lake and transitions to wetland (Figure 1). This geomorphology is consistent with other subglacial and proglacial meltwater channels, which is the likely origin of Chub Lake. Chub Creek is a small tributary of the Cannon River and drains Chub Lake through the wetland area to the south (Figure 1).

Surficial geology in the catchment includes Quaternary deposits associated with the Des Moines Lobe of the Laurentide Ice Sheet draping Ordovician aged sedimentary units including the St. Peter Sandstone and Prairie du Chien (PdC) Group carbonates (Figure 1). The contact between the Prairie du Chien Group and the St. Peter Sandstone occurs some ∼25 m below the base of the Chub Lake basin (Figure 1). Artesian groundwater springs are a common feature of this contact (Balaban et al., 1990) throughout the region, and given the lack of obvious surface water inputs, Chub Lake is likely fed primarily by groundwater. Although, aerial images from the site suggest there may be an emphemeral stream that occasionally delivers surface water to the lake from the northwest (Supplemental 1). Groundwater in this region is characterized as Ca-Mg bicarbonate type with a relatively high iron content (Palen, 1990).

Soil types in the catchment are a mixture of Lester Loam, Merton Silt Loam, and Maxcreek Silty Clay Loam (Soil Survey Staff, 2019). The parent material of Lester Loam is loam rich glacial till, while Merton Silt Loam and Maxcreek Silt Loam both form over loess (Soil Survey Staff, 2019). Lester Loam and Merton Silt Loam contain up to 30% calcium carbonate, while Maxcreek Silty Clay Loam is typically no more than 5% calcium carbonate. Till deposits in the lake catchment are associated with the Des Moines Lobe and are known to contain clasts of the calcareous Cretaceous Pierre Shale (Lusardi et al., 2011).

The modern landscape in the Chub Lake catchment is primarily agricultural croplands, neighborhoods, and motorways. Prior to European colonization (∼1850), Chub lake was surrounded by a mix of oak prairie and wet prairie (MN Dept. of Natural Resources, 2022). Mean annual temperature is 8.1°C and the mean annual precipitation is 103.8 cm (for time period of 1991–2020; Arguez et al., 2012; US Department of Commerce, 2020).

Core collection

A set of continuous, overlapping sediment cores with a composite depth of 9.25 m were collected from the center of Chub Lake in January 2019 (Figure 1). Sediment was collected using a polycarbonate piston, modified Bolivia, or modified Livingston coring device (Wright et al., 1984). Sediment cores were split shortly after collection and then scanned using a Geotek Core Imaging System to acquire high resolution images. Core collection and processing was conducted in collaboration with staff at the Continental Scientific Drilling Facility (CSD) at the University of Minnesota. Unless otherwise noted, all analysis reported here was conducted at CSD.

Sediment descriptions

Sediments from Chub Lake were described and classified based on the relative abundance of biogenic, clastic, and chemical components (Schnurrenberger et al., 2003). Major (50–25%) and minor (15–25%) sedimentary components were recorded based on analysis of smear slides using a standard petrographic microscope. For much of the core, biogenic sediment in Chub Lake is dominated by amorphous algal matter, with heavily diatomaceous layers. Toward the top of the core, plant macrofossils, sponge spicules, and animal remains such as insect fragments become abundant, and are also classified as biogenic sediments. Clastic sediment refers to sediment dominated by clay to silt sized sediment derived from weathering outside of the lake and delivered into the lake through erosion or eolian processes.

Chemical sediments are dominated by authigenic carbonate. Biogenic and chemical sediment often co-occur where authigenic carbonate encrusts amorphous algal matter. A dominate majority of carbonate grains identified in smear slides exhibited this encrusting behavior or were identified as authigenic by their euhedral nature. There was a lack of persistent and obvious detrital carbonate grains identified in the sediment, which can typically be identified as larger, anhedral, standalone particles.

Age model

Eighteen radiocarbon dates were acquired using accelerator mass spectrometry (AMS) on pollen and charcoal samples from Chub Lake. All AMS dating was conducted at the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Livermore National Laboratory, Livermore, California, USA. Dates from pollen and charcoal samples were processed using the Bayesian based software package BACON in R (See Supplemental 2; Blaauw and Christen, 2011). The software transforms measured radiocarbon dates to probability fields using a Markov-Chain Monte Carlo over ∼20 million iterations. The age of sediments between radiocarbon dates are estimated using piece-wise linear regression on the age-depth data divided into sixteen 50 cm thick sections. Accumulation rates are assumed to be constant within each section. Additional discussion of the age model is included in the results section.

Loss-on-ignition

We quantified bulk sediment composition using loss-on-ignition (LOI). LOI estimates the weight percent of water, organic matter, carbonates, and incombustible material (typically clastic minerals and biogenic silica) in sediment samples through combustion at different temperatures (Heiri et al., 2001; Vereș, 2002). Samples are first dried at 105°C to remove water and to determine initial dry sediment mass. Mass-loss following a first round of combustion for 4 h at 550°C is used to determine the weight percent organic material in the sample. A second round of combustion for 2 h at 1000°C estimates the weight percent carbonate (Dean, 1974; Vereș, 2002). The weight of the residual material after the final combustion relative to the initial dry weight is referred to generally as weight percent incombustible material.

Carbon and nitrogen

Total carbon (C), total nitrogen (N), and the stable isotopic composition of carbon (δ¹³C) for organic matter in Chub Lake sediments were measured using a Costech (Valencia, California, USA) CHNS Analyzer with a Delta V Advantage Isotope Ratio Mass Spectrometer (ThermoFisher, Bremen, Germany) at St. Olaf College. Prior to analysis, between 2 and 5 mg of freeze dried sediment was soaked in 4 mL of 1N HCl for 24 h to remove carbonate minerals and then rinsed in DI water 2–4 times prior to freeze drying.

The ratio of carbon to nitrogen (C:N) is useful for determining the source of organic matter in sediment cores (Contreras et al., 2018; Kemp et al., 1977). Algal organic material typically has C:N ratios between 4 and 9 (Contreras et al., 2018; Hyne, 1978; Kemp et al., 1977), while vascular plants (terrestrial and aquatic) have C:N ratios between 15 and 25 (Hyne, 1978). Woody plants have much higher ratios, often between 25 and 55 (Hyne, 1978; LaZerte, 1983). C:N is subject to alteration by diagenesis (Meyers and Ishiwatari, 1993). For example, slight decreases in C:N across the sediment water interface are sometimes related to the microbial immobilization of nitrogenous material (Kemp et al., 1977; Meyers and Ishiwatari, 1993).

Carbon isotopic composition (δ¹³C) of organic matter in sediment can be useful for determining the lake’s water source, as well as the amount of internal isotopic fractionation (Lang and Marshall, 2004; LaZerte, 1983). Ground water sourced from carbonate bedrock generally has a δ¹³C of between −3‰ and 3‰ (Lang and Marshall, 2004). However, all plants preferentially utilize carbon-12, leading to a −20‰ fractionation of δ¹³C_OM in C3 plants and a −4‰ to −6‰ fractionation in C4 plants, relative to the source water (Meyers and Lallier-Vergès, 1999). Increases in δ¹³C of organic matter may relate to longer water residence time (Dean and Schwalb, 2002), or periods of increased productivity (Meyers and Lallier-Vergès, 1999). In shallow conditions, depletion of δ¹³C may indicate a population increase in aquatic plants with emergent leaves.

Biogenic silica

Biogenic silica (BSi; reported as a weight percent) was measured using the wet alkaline digestion method described in full by Conley and Schelske (2001). Silica was extracted from 30 mg sediment samples using 1% Na₂CO₃ solution and measured colorimetrically on 3, 4, and 5 h digests.

Diatoms are the most common form of BSi in lake sediments (Qiu et al., 1993; Struyf and Conley, 2009), and BSi is interpreted primarily as a lake productivity proxy (where higher BSi indicates higher productivity and vice versa, see Qiu et al. (1993). In shallow environments, BSi can also be sourced from terrestrial run-off in the form of phytoliths of terrestrial plant matter (Struyf and Conley, 2009). Some studies have reported a gradual decline in BSi with depth, indicating that dissolution of deposited BSi may influence lacustrine records (Struyf and Conley, 2009).

Charcoal

Charcoal was extracted from 1 cm³ sub-samples collected at 10 cm intervals of the core. Samples were soaked for 24–48 h in 10% KOH solution and sieved with a 180 µm sieve. Sieved material was spread over a gridded petri dish and analyzed for particle area (mm²) and shape (length:width ratio) using a 20× magnification dissecting microscope. Charcoal data is reported as the concentration (mm²cm⁻²) and the influx (mm²cm⁻²yr⁻¹) based on age model data.

Charcoal data is interpreted primarily as a moisture proxy, with increases in precipitation leading to larger fuel loads and more severe burns in prairies (Umbanhowar, 1996). This relationship between fire severity and available moisture is not as clear in forests.

Elemental composition: Scanning XRF

To characterize the elemental composition of sediments from Chub Lake, cores were scanned using the ITRAX X-Ray Fluorescence (XRF) Corescanner at the Large Lakes Observatory at University of Minnesota Duluth at a resolution of 10 cm. The ITRAX XRF instrument produces elemental counts for most elements with molecular weights ranging between Al and Pb. Raw element counts were converted to center-log ratios (clr) using the “rgr” package in R (Garrett, 2013), to help correct for non-linearity (Davies et al., 2015; Weltje and Tjallingii, 2008; Żarczyński et al., 2019). Scanning XRF data is widely applied to the study of lacustrine sediments (Brown et al., 2007; Davies et al., 2015; Kylander et al., 2011; Martín-Puertas et al., 2011) and we employ methods consistent with several recent studies (McLauchlan et al., 2019; Pleskot et al., 2018; Żarczyński et al., 2019).

We focused our analysis on Al, Si, S, K, Ti, Ca, Mn, and Fe because these elements best constrain changes in sediment composition which reflect in-lake and catchment processes. Al, K, and Ti are abundant in clastic minerals and represent detrital input to the lake (Davies et al., 2015), either from terrestrial run-off or aeolian inputs (Davies et al., 2015; Kylander et al., 2011). Si is also abundant in clastic minerals (Kylander et al., 2011; Martín-Puertas et al., 2011), but also in biogenic silica (Davies et al., 2015).

Fe and Mn deposition is generally related to the redox conditions of bottom waters and the sediment water interface (Kylander et al., 2011). Both Fe and Mn are soluble in anoxic waters, but Mn to a greater degree (Davies et al., 2015). Peaks in the ratio of Mn to Fe are often indicative of the onset of reducing conditions in lake environments (Davies et al., 2015). However, both elements are also common in detrital material, and in cases where there is little covariance between Mn and Fe, one or both elements may be related to detrital sources (Davies et al., 2015).

Ca abundance is tied to carbonate minerals, which can be either authigenic or clastic in origin (Davies et al., 2015; Myrbo, 2012). Authigenic carbonate minerals form in lakes, typically related to groundwater inflow, alkalinity, and lake productivity (Dean and Megard, 1993; Dean and Schwalb, 2002; Myrbo, 2012; Shapley et al., 2005). Carbonate minerals also erode into the lake, and can be sourced carbonate rich soils, sediments, or bedrock within the catchment.

To quantify and interpret covariance of XRF data, principal component analysis (PCA) and a k-means clustering analysis were performed on the center-log-ratios of the eight elements of interest (Al, Si, S, K, Ti, Ca, Mn, and Fe). PCA loads individual variables along principal components, which together describe variation in multidimensional data. Positive loading along a principal component for a given variable indicates a positive relationship, where higher principal component scores are associated with higher values for a given variable (and vice versa for negative scores). K-means clustering evaluates the distance between individual observations and mean values for data clusters. For a given number of clusters (user defined), cluster means are optimized to reduce the residual differences between means and individual observations within a cluster. We use PCA to describe and interpret geochemical variation in our dataset and k-means clustering to characterize packages of sediment with shared geochemical traits. The “FactoMineR” package in R was used to run PCAs (Lê et al., 2008), and the “Factoextra” package was used to visualize PCA results (Kassambara and Mundt, 2020; Supplemental 3). The “Cluster” package in R was used to conduct k-means clustering analysis (Maechler et al., 2019; Supplemental 3).

Results

The Holocene record from Chub Lake contains five distinct sedimentary intervals. The boundary between each interval corresponds to either a change in sedimentology, a shift in geochemical characteristics determined through k-cluster analysis, or to some combination of both. Below, we first present the results of our age model. Next, we report sediment classification by time interval along with results of k-cluster analyses on elemental data. Finally, changes to all remaining proxy records within each time interval are reported chronologically.

Age model

Of the 18 radiocarbon dates collected (Table 1), 15 demonstrate relatively consistent deposition of sediments over time (Figure 2). However, three anomalous dates likely result from slump events where sediments were re-mobilized. The anomalous dates would require that older sediments be deposited on top of younger sediments, which is not possible without sediment re-mobilization (Table 1, Figure 2). This section of the core contains two sets of distinct normally graded beds (Figure 3), which support the interpretation that these were mass re-mobilization of shallower sediment into deeper water. The normally graded beds were modeled as two instantaneous deposits that occur around ∼ 8100 and 8200 B.P. (Figure 2; Supplemental 1). Sedimentation rates are roughly consistent from 8100 B.P. through the present, with slower sedimentation in the early Holocene (9700–8200 B.P.; Figure 2). Slower early Holocene accumulation rates are consistent with the sediment record from Sharkey Lake, which is just 15 km West of Chub Lake (Camill et al., 2003).

Table 1.

AMS pollen dates – anomalous dates denoted with an asterisk.

Core ID	Source	Composite depth (cm)	Radiocarbon age (years B.P.)	Error
Carl2-Chub19-1B-1B	Pollen	100.5	1200	35
Carl2-Chub19-1A-2B	Charcoal	208.5	2410	35
Carl2-Chub19-1B-2B	Pollen	301.0	3140	30
Carl2-Chub19-1B-3B	Charcoal	398.4	3730	35
Carl2-Chub19-1B-4B	Charcoal	500.6	4490	30
Carl2-Chub19-1B-4B	Charcoal	505.6	4535	30
Carl2-Chub19-1B-4B	Charcoal	611.1	4935	30
Carl2-Chub19-1A-6L	Charcoal	655.5	5460	35
Carl2-Chub19-1A-6L	Charcoal	660.5	5565	30
Carl2-Chub19-1A-6L	Pollen	703.5	5980	30
Carl2-Chub19-1B-5L	Pollen	721.2	6570	40
Carl2-Chub19-1A-7L	Pollen	745.6	6555	30
Carl2-Chub19-1A-7L	Pollen	772.1	8785*	35
Carl2-Chub19-1A-7L	Pollen	776.6	8770*	35
Carl2-Chub19-1A-7L	Pollen	779.0	8940*	35
Carl2-Chub19-1B-7L	Charcoal	820.8	7820	35
Carl2-Chub19-1B-7L	Charcoal	825.3	8585	35
Carl2-Chub19-1B-7L	Charcoal	830.3	9140	35

Figure 2.

An age model constructed using a set of 15 radiocarbon AMS pollen dates calibrated and processed using the BACON software package in R. The series of age models is represented as the light gray field (Blaauw and Christen, 2011). The upper dark gray line represents the age model with the maximum age per depth, while the lower gray line represents the age model with the minimum age per depth. The red line represents the median of these values and is the value represented by “Cal Age B.P.” in this paper. Gray bands indicate the location of modeled slump deposits.

Figure 3.

Selected split core images from Chub lake showing representative sections of each lake time zone. Slump deposits in the early Holocene, which were removed from the age model, are marked in red squares.

Sediment descriptions

Basal sediment in Chub Lake (Earliest Holocene; ∼11,000–9700 B.P.; Figure 3) is light gray to black laminated silty clay. The early Holocene (9700–8200 B.P.) is light brown to brown, thin bedded (3–10 cm scale bedding) sediment variously composed of near equal parts authigenic carbonate, clays, and amorphous algal material. Figure 4 shows images of these components. In the Mid-Holocene (8200–4250 B.P.) sediments are light brown to tan, thin bedded, diatomaceous oozes throughout, with increased abundance of authigenic carbonates in light tan beds (Figure 4). The Mid-to-Late-Holocene (4250–2300 B.P.) is light brown to tan, indistinct to massively bedded, carbonate rich peat with visible aquatic plant macrophytes. The Late-Holocene (2300 B.P. to present) is a brown to dark brown, thin bedded peat with minor authigenic carbonates early in the section replaced with minor clays in more recent sediments.

Figure 4.

Smear slide images of Chub Lake sediments. Photos 1a (taken in plane polarized light) and 1b (taken in cross polarized light), taken from the same field of view, show sediment deposited in the Mid-to-Late-Holocene, and include authigenic carbonate, amorphous algal matter, charcoal, and diatoms. Photo 2 (left quarter in plane polarized light, right three quarters in cross polarized light), shows sediment deposited in the Mid-Holocene, and include richly diatomaceous sediment, charcoal, and amorphous organic matter. Photo 3 (right half in plane polarized light, left half in cross polarized light), shows sediment deposited during the Early Holocene slump deposits, and includes many clastic minerals. Examples of these sedimentary components are emphasized by arrows. Orange arrows indicate amorphous algal matter, blue arrows indicate diatoms, white arrows indicate authigenic carbonate minerals, purple arrows indicate charcoal, and pink arrows indicate clastic minerals.

PCA and K-clusters

The first principal component (PC1) in our PCA describes 52.4% of variance and the second principal component (PC2) describes 35.5% of variance (Figure 5). For PC1, Si, K, Ti, and Fe have positive loadings and Ca, Mn, S, and Al load negatively (Figure 5). For PC2, Mn Al, Fe, and Ti all load positively and Ca, S, Si, and K load negatively (Figure 5). Points in our PCA analysis were classified according to k-means clustering (referred to here as k-clusters). In K-cluster 1, points are defined by strongly positive PC2 scores related to increased Mn, Fe, and Al. Points in k-cluster 2 are defined by relatively positive PC1 scores, related to increased Si, K, and Ti. Points in k-cluster 3 are primarily defined by negative PC1 scores, driven by increased Ca.

Figure 5.

Results of principal component analysis (PCA) of full sediment cores for Chub Lake. Panel at left shows the bi-plots of variable loadings (top left) and principal component (PC) scores (lower left) for PC1 and PC2. Panels at right show PC1 and PC2 scores plotted against age. Black horizontal lines indicate the time period boundaries established in the results section via k-cluster and sediment composition analysis. Point shading corresponds to k-cluster groupings shown in the legend in the lower.

We segmented the core into five distinct time zones on the basis of k-cluster analysis and sedimentology (Figure 3). Our K-cluster analysis results divide the record into four temporal zones, each dominated by a unique k-cluster (see lower panels in Figure 5). The earliest Holocene (11,000–9700 B.P.) was not sampled for XRF data, and is not included in our k-cluster analysis – although we do include this time period in our discussion based on sedimentology. The Late-Holocene (2300 B.P. to present) is primarily controlled by k-cluster 1. The Mid-Holocene (8200–4250 B.P.) is largely controlled by k-cluster 2. The early Holocene (9700–8200 B.P.) and the Mid-to-Late-Holocene (4250–2300 B.P.) are controlled by k-cluster 3.

Earliest Holocene 11,000–9700 B.P

The Earliest Holocene boundary is clearly defined through changes in sediment composition and structure relative to other core sections (Figure 3). Smear slide analysis shows clastic rich sediment likely made up of glacial clays. XRF and LOI analyses were conducted at the rough boundary between the Earliest Holocene and the Early Holocene. XRF data collected at ∼9760 B.P. is high in Si, Ti, and K (Figure 6), and falls into k-cluster 2 (Figure 5). LOI data, collected at ∼9700 B.P., shows ∼80 wt% incombustible material, and ∼20 wt% carbonate (Figure 7), with very little organic matter (<5 wt%). No other proxy data was collected in this zone.

Figure 6.

Elemental data from Chub Lake. Data displayed are center-log ratios of elemental counts. Black lines indicate the time period boundaries established in the results section via k-cluster and sediment composition analysis.

Figure 7.

Loss-on-Ignition and grain size data from Chub Lake. Black lines indicate the time period boundaries established in the results section via k-cluster and sediment composition analysis.

Early Holocene 9700–8200 B.P

The Early Holocene is dominated by k-cluster 1 (Figure 5) and the top boundary of this section occurs just above the slump deposits in the core, which are interpreted as a mark of rapid lake level fall (boundary occurs at 770 cm depth; age is ∼8200 B.P.). This section is notable because nearly all elemental data shift dramatically at the onset of this time period (Figure 6). Clastic signatures (Si, Ti, and K) fall rapidly following the onset of this period. Meanwhile, Ca values rise abruptly, as do Mn, S, and Fe to a lesser extent. Charcoal concentration peaks in the second half of this time zone (Figure 7). δ¹³C_OM is most depleted in this section with a range from −24.5‰ around 8600 B.P. to −26.5‰ in the slump deposits (Figure 7). The C:N ratio increases from a low of 10.5 to a peak of 11.5 at the end of this zone (Figure 7). We note that throughout the core, C:N ratio is variable and fluctuates between 9.5 and 11.5. LOI data in this zone is roughly an equal mix of carbonate, organics, and incombustible material, with no major peaks in any group (Figure 7). BSi is lowest in this section, with a range from 0.8% to 1.0% (Figure 7). Median grain size remains steady at ∼12 µm throughout this zone (Figure 7).

Mid-Holocene 8200–4250 B.P

The Mid-Holocene is broadly defined by k-cluster 2 (Figure 5), although some data points are designated as k-cluster 3 near the top of this interval (beginning ∼4600 kyB.P.). K-clusters do not correspond exactly with the upper boundary of this zone because it is defined by the onset of visible fibrous macro-organics (Figure 3), rather than by k-cluster transition. K-cluster 2 is largely controlled by variance in Si, with influence from K and Ti. Early Si, Ti, K, and elevated Fe likely represent detrital input during this period, but at 6700 B.P., BSi values peak suggesting Si no longer dominantly detrital in origin at that time. BSi remains high, though variable (primarily between 3 and 9 wt%), throughout the remainder of this time period (Figure 7). Ca also experiences high variance during this period, with alternating peaks and troughs on 1000-year time scales. S experiences similar peaks and troughs, though with a smaller magnitude (Figure 6). LOI data also shows variable carbonate across this period. δ¹³C_OM becomes steadily more enriched across this period, with discrete peaks that line up with the onset of elevated Ca values as well as peaks in charcoal concentration (Figure 7). The C:N ratio is variable throughout this zone, from minimums of ∼10 to maximums of ∼12. Weight percent incombustible material accounts for >60% of the sediment composition, and organic material accounts for >20% of the sediment composition (Figure 7). Grain size is variable but increasing across this time period, from ∼12 to ∼23 µm (Figure 7).

Mid-to-Late-Holocene 4250–2300 B.P

The majority of the Mid-to-Late-Holocene is defined by k-cluster 3 (Figure 5). K-cluster 3 is most strongly controlled by Ca and S, with inputs from Mn and Al as well. XRF data show a prolonged Ca peak for the entirety of this period (Figure 6), while Si, Ti, K, and Fe values fall. S values peak here as well. BSi values are between 3.25% and 8.99% for the majority of this time period, but decrease sharply at 2700 B.P. (Figure 7). δ¹³C_OM enrichment, which steadily increases from a low at ∼8.2 kyB.P. to a peak of -10 ‰, around 3 kyB.P. before collapsing back to −20‰ at the end of this period (Figure 7). Charcoal concentration and influx show a step-wise decrease in this interval (with a relatively abrupt decline in both around 3 kyB.P.). C:N is variable, but increases at the end of the period (Figure 7). Grain size generally coarsens upward in this interval to a peak (∼40 µm) near 3 kyB.P. (Figure 7).

The upper boundary of this time period is not defined by a change in k-cluster but rather a shift in sedimentology. LOI data show an abrupt shift from high carbonate to high organics across this boundary, and incombustible material switches from gradually decreasing to gradually increasing across the boundary (Figure 7). This change is also apparent in core images, which show a similar shift from light (carbonate rich) to dark (organic rich) colored sediment packages across the upper boundary (Figure 3).

Late-Holocene 2300 B.P. to present

The Late-Holocene is best described by k-cluster 1, although the start of the zone at 2300 B.P. is classified as k-cluster 3. K-cluster 1 is controlled by primarily by variance in Mn and Fe (Figure 5). Fe, Mn, and Al are elevated above mid to early Holocene levels, and K and Ti values begin low but increase steadily during this interval. S values steadily decrease over this interval (Figure 6). The Late-Holocene is distinct for having the highest weight percent of organic matter in the core, peaking about 65% and the lowest weight percent carbonate (generally below 20%; Figure 7). Variation in Ca largely follows trends observed in weight percent carbonate from LOI in the Late-Holocene (Figure 6). Grain size falls from Mid-to-Late-Holocene peaks to a range of ∼7 to ∼19 µm (Figure 7). BSi is relatively low throughout this zone, but rises to an abrupt peak of 4.2. δ¹³C_OM starts relatively depleted in this section of the core, with values around −20‰. It increasing to a peak of −15‰ around 1000 B.P., before depleting back to the −20‰ range by the end of the period. The C:N ratio is variable throughout this zone, from minimums of ∼10 to maximums of ∼12 (Figure 7). Both charcoal influx and concentration fall to a minimum during this period (Figure 7).

Discussion

The sediment record from Chub Lake captures a relatively high-resolution response of this shallow lake system to changes in regional climate throughout the Holocene. Below, we first describe how changes in regional climate appear to be recorded by variation in Chub Lake sedimentation by summarizing change in the proxies presented in this study. Next, we focus our discussion on the occurrence of carbonate minerals in Chub Lake sediments and evaluate their utility as a moisture proxy. Finally, we present a hypothesis for the abrupt change in deposition that occurs ∼2300 B.P., where there is a sharp increase in organic rich sedimentation.

Response of lake system to regional climate variation

As described in the results above, Chub Lake sedimentation can be characterized by intervals between boundaries set at 9700 B.P., 8200 B.P., 4250 B.P., and 2300 B.P. The boundaries in our study, which were determined through a combination of k-cluster analysis and sedimentology, align well with boundaries established for regional climate shifts designated by other Holocene lake sediment records and palynology (Camill et al., 2003, 2012; Shuman and Marsicek, 2016; Wright, 1992).

Cool and humid conditions characterize the early Holocene (11,400–8200 B.P.) climate across the Midwest due in large part to the waning influence of the retreating Laurentide Ice Sheet (Shuman and Marsicek, 2016). During this time period, Chub Lake sedimentation is characterized within two intervals. The first (∼11,000–9700 B.P.) is represented by clays deposited early in the development of the Chub Lake basin that were not closely evaluated by this study. Sediments above the glacial clays (9700–8200 B.P.) are roughly equal parts carbonate, amorphous organic matter, and fine-grained clastic minerals (variable, but between 20 and 40 wt% each) with very little biogenic silica (<1 wt%) and relatively low influx of charcoal (values near 0, see Figures 6 and 7). This is consistent with a relatively cool, low productivity lake (low BSi) in a humid climate with forested conditions (low charcoal influx; see also Camill et al., 2003; Geiss et al., 2003). Notably, the top of this interval is represented in Chub Lake by a sequence of slump deposits (Figure 3) that occur around ∼8200 B.P. that likely represent a transient lake-level fall. These slump deposits are likely associated with abrupt climate impacts of the 8.2 ka event in the North Atlantic (Alley et al., 1997), thought to be caused by the catastrophic draining of glacial Lake Agassiz, that led to aridity in the mid-continent (e.g. see; Dean et al., 2002).

Between the collapse of the Laurentide Ice Sheet in ∼8200 B.P. and a shift to cooler and more humid conditions around ∼4500 B.P., regional climate was relatively warm and arid (Shuman and Marsicek, 2016; Wanner et al., 2011) during the mid-Holocene. Prairie ecosystems expanded and pushed eastward into much of the area surrounding Chub Lake (Geiss et al., 2003; Umbanhowar, 1996; Wright, 1992). Chub Lake sediments during this period have characteristically low, but variable, amounts of carbonate (varies between ∼7 and 25 wt%), steadily increasing organic matter (from ∼15 to 30 wt%), and high clastics (∼50–75 wt%) and BSi (Figure 7). Notably, intervals of higher carbonate occur when the abundance of clastic minerals is low, and vice-versa, oscillating on a quasi-1000 year cycle between ∼8200 and 2500 (a feature we discuss in more detail below, see PC1 in Figure 5). Fine median grain size throughout this period suggests eolian clastics transport, as in an arid landscape (Geiss et al., 2003). The mid-Holocene in Chub Lake also sees an increase in BSi and charcoal influx, along with a steady increase in the δ¹³C_OM (Figure 7). Around 4500 B.P. regional climates cool and eventually begin to become more humid (Camill et al., 2003). This boundary in Chub Lake roughly corresponds to a shift toward more carbonate rich sedimentation, lower concentrations of clastic minerals, and continued increases in both organic matter and the δ¹³C_OM (Figure 7).

Changes in sedimentation within Chub Lake from 9700 B.P. to ∼2300 B.P. suggest that changes in regional climate are broadly captured by changes in lake sedimentation, consistent with observations from other regional proxy records (Camill et al., 2003; Dean and Schwalb, 2002; Geiss et al., 2003; Wright, 1992). However, variability in several key sediment proxies suggest a higher resolution response within Chub Lake that is sensitive to variation in local hydroclimate. Sedimentation after 2300 B.P. in Chub Lake is dominated by organic matter and many of the observed relationships between climate and sedimentation seem to either change or become less apparent. Below, we discuss each of these aspects of the Chub Lake record in more detail.

Carbonate deposition as a moisture proxy

We propose that authigenic carbonate mineral content of Chub Lake sediments, as recorded by both the weight percent carbonate (from LOI) and the elemental Ca record (from scanning XRF), represents a moisture proxy from the early Holocene up until ∼2500 B.P. This moisture proxy is based on the relationship between carbonate deposition and groundwater recharge.

Groundwater recharge rates to lakes is dominated by regional hydroclimate (Nygren et al., 2021; Shapley et al., 2009). At low moisture availability and low groundwater inflow, summer authigenic carbonate deposition becomes limited by Ca availability in lake waters (Shapley et al., 2005). Small amounts of authigenic carbonate may still deposit under low moisture conditions (here, for Chub Lake ∼10 wt% in LOI data; see Figure 7 during the Mid-Holocene). When groundwater recharge increases under more humid hydroclimate, the increased influx of Ca from groundwater into Chub Lake leads to increased deposition of authigenic carbonate, which proceeds until Ca is limited once again (Shapley et al., 2005).

Authigenic carbonate mineral formation in lakes typically occurs during warm summer months in response to the photosynthetic drawdown of CO₂ (Dean and Schwalb, 2002; Myrbo, 2012; Shapley et al., 2005). The subsequent deposition of carbonate minerals along with algal organic matter results in gyttja that is common in many lakes in Minnesota (Dean, 1999), including Chub Lake throughout the early to Late-Holocene (Figures 3, 6, and 7). While many factors influence carbonate mineral formation, deposition, and preservation in lacustrine systems (Almendinger and Leete, 1998; Myrbo, 2012; Nelson et al., 2011), the availability of Ca in lake waters can act as an important limitation on authigenic carbonate formation under certain conditions (Shapley et al., 2005).

Elevated levels of Ca in lake water is common for groundwater fed lakes in regions with near-surface carbonate-rich bedrock or sediments (Horvatinčić et al., 2018; Li et al., 2021; Metcalfe et al., 2022; Valero-Garcés et al., 2014). In Minnesota, many lakes are formed within calcareous glacial drift and/or Cambrian-Ordovician aged carbonate bedrock units (Dean, 1999). Chub Lake is situated within surficial sediments associated with the calcareous Des Moines Lobe and occurs at the contact between the Prairie du Chien Group, an Ordovician carbonate deposit, and the St. Peter Sandstone (Figure 1). The Prairie du Chien-St. Peter contact is well-known to be a source of groundwater springs throughout the region, and groundwater from the Prairie du Chien aquifer likely supplies water to Chub Lake. Taken together, we make the conservation assumption that Ca in Chub Lake waters is primarily delivered through groundwater recharge.

If the rate of carbonate mineral formation (and preservation in sediments) outpaces delivery of new calcium cations into the lake through groundwater recharge, then over time the rate of authigenic carbonate mineral formation will become rate-limited by the availability of Ca (Shapley et al., 2005). This scenario requires a lake water residence time that is sufficiently long to allow Ca levels in the lake to be depleted. For Chub Lake, the progressive enrichment of the δ¹³C_OM from ∼9000 to ∼2500 B.P. suggests that Chub Lake was hydrologically closed, with long lake water residence times that can lead to the progressive enrichment of δ¹³C_OM observed in the core (Dean and Schwalb, 2002; Kelts, 1988; Li et al., 2021; Figure 7).

The two intervals with the highest Ca/carbonate abundance within Chub Lake occur during the Early (9700–8200 B.P.) and Mid-to-Late-Holocene (4250–2300 B.P.; Figures 6 and 7; wt% carbonate varies ∼20–40), periods that are well-described as humid and cool across the mid-continent (Shuman and Marsicek, 2016; Williams et al., 2009) and from other lake records nearby to Chub Lake (Camill et al., 2003; Geiss et al., 2003). Meanwhile, the warm and dry middle Holocene (8200–4250 B.P.; Shuman and Marsicek, 2016; Williams et al., 2009) has a lower abundance of Ca/carbonate (Figures 6 and 7; wt% carbonate varies from ∼5 to 20) consistent with a depositional model where carbonate mineral formation is limited by recharge of groundwater sourced Ca. Notably, the shift to lower carbonate mineral content occurs just above the slump deposits at ∼8200 B.P. that we interpret as a decrease in lake level. Lower carbonate mineral formation following this transition is consistent with other regional records documenting shifts to more arid climates following the 8.2 kya cold event in the North Atlantic (Dean et al., 2002). Median grain size throughout this period is notably low, suggesting eolian deposition from an arid landscape (Geiss et al., 2003). Increases in grain size may suggest increased surficial water inflow during periods of elevated moisture availability.

During the warm and dry middle Holocene, there appear to be at least three intervals of increased moisture availability, as evidenced by increases in the Ca and carbonate records, that occur on quasi 1000 year cycles (Figures 6 and 7). These transient increases in carbonate production correspond to relative peaks in several other proxies from Chub Lake that are consistent with increased moisture. For instance, peaks in charcoal influx (Figure 7) at 7500, 6800, and 5500 B.P. correspond to relative peaks in Ca and wt% carbonate (Figures 6 and 7). Charcoal influx generally indicates fire severity in the catchment, and tends to increase as a result of moisture-driven increases in biomass production on land that increase the available fuel stocks for fires within prairie environments (Camill et al., 2003; Umbanhowar et al., 2009). In addition, superimposed on the long term enrichment of the δ¹³C_OM, transient increases in the δ¹³C_OM may directly capture increased groundwater influx (assuming carbonate rich groundwaters have enriched δ¹³C). Peaks in δ¹³C_OM at 8200, 6800, and 5500 B.P. align well with records of Ca deposition during the Mid-Holocene (Figures 6 and 7), perhaps due to a sudden influx of enriched groundwater (Lang and Marshall, 2004). Peaks in S deposition also align with Ca. In varved carbonate lakes, iron-sulfide deposits represent winter decomposition of organic materials at the anoxic sediment surface (Kelts and Hsü, 1978). Here, peaks in S likely represent relative peaks in productivity and organic mater availability, resulting from increased humidity. Together, these occurrences support the idea that Ca in Chub Lake sediments is a proxy for regional hydroclimate.

Other work has pointed to a potential ∼1000 year variability of the Holocene climate (Bond et al., 1997, 2001; Denton and Karlén, 1973; Wanner et al., 2011). Specifically, other research has established several “cold relapses” that punctuate the stable, warm Holocene climate and which support our interpretations of cold periods during the warm, arid Mid- and Mid-to-Late-Holocene. These cycles were first established in ice rafted debris records from the North Atlantic (Bond et al., 1997, 2001), but have since been noted in pollen records (Viau et al., 2002; Willard et al., 2005), cosmogenic nuclides (Hu et al., 2003), and δ¹⁸O records (Alley, 2000; Grafenstein et al., 1999), across Europe, North Asia, and North America (Wanner et al., 2011). Evidence for these cycles show high spatial variability and analysis across global records (Wanner et al., 2011) shows cycles slightly offset from moisture peaks at Chub Lake. However, Bond cycles identified in pollen records and ice rafted debris from North America are more closely aligned with Chub Lake’s moisture peaks (Bond et al., 2001; Viau et al., 2002). Bond et al. (2001) describes decreases in solar irradiance at 2700, 6300, 7400, and 8300 BP, and Viau et al. (2002) describes transitions to cooler climates at 4030, 6700, and 8100 BP. These dates correspond with Ca-peaks in Chub lake at 4250, 6800, 7500, and 8200 BP.

Depositional shift at 2500 B.P

Prior to ∼2500 B.P. Chub Lake was likely a relatively shallow, hydrologically closed lake system with limited outflow. This interpretation is anchored in two key characteristics of Chub Lake and its sediment record. First, the Chub Lake sediment record has no indication of any laminated intervals – which are common in deeper lakes that develop stronger seasonal stratification. The water column in Chub Lake then has likely been relatively well mixed for the duration of the Holocene. Second, lake outflow was likely limited during this time period. Today, water outflow to Chub Creek drains the lake to the south. However, beginning in the Early Holocene when sedimentation began, the Chub Lake basin would have been ∼9 m deeper than present (the total thickness of sediment recovered). The increased accommodation space, combined with the understanding that Chub Lake has remained relatively shallow, suggests that the water level in Chub Lake may have been lower than the levee for present day Chub Creek for much of the Holocene. Limited outflow, as discussed above, increases water residence time and can be responsible for the progressive enrichment of carbon isotopes in organic matter as reported here (Figure 7; Dean and Schwalb, 2002; Kelts, 1988; Li et al., 2021).

Beginning around 2500 B.P. there is a large shift in many of the proxy records reported here, suggesting a major change in the depositional regime of the lake. We interpret this shift to represent a relatively rapid shallowing accompanied by increased outflow. Paradoxically, the combination of increased humidity and moisture availability in the Late-Holocene (Camill et al., 2003; Shuman and Marsicek, 2016) along with decreasing accommodation space (from basin infilling) raised relative base level to a point where we propose Chub Creek levees were breached and outflow from the lake increased, ultimately leading to a very shallow lake system with increased hydrologic inflow (due to more humid regional hydroclimate) and outflow (via Chub Creek) together reducing lake water residence time.

Around 2500 B.P., there is a large decrease in Ca/carbonate deposition (Figures 6 and 7) and a depletion (decrease) in δ¹³C_OM. The depletion of δ¹³C_OM likely indicates reduced residence times for lake water, and may also indicate more incorporation of plant material from species that draw carbon directly from the atmosphere (typical of near-shore or very shallow environment species). Median grain size also gradually increases before 2500 B.P, to a peak at 2900 B.P. Increased lake energy from new surficial inflow and outflow and increased moisture availability may account for the peak in median grain size during the Mid-to-Late-Holocene (Roeser et al., 2016). Importantly, shorter residence time for water in Chub Lake would lead to a decoupling of the relationship between moisture availability (which is increased at this time) and carbonate mineral formation (which declines) due to loss of Ca in the system to outflow (Shapley et al., 2005). So, whereas decreases in carbonate mineral abundance prior to 2500 B.P. in Chub Lake is indicative of more arid climate conditions, the change in lake regime disrupts this proxy for the Late-Holocene.

As carbonate mineral abundance decreases around 2500 B.P. in a more humid climate, there is a large increase in sedimentary organic matter and a gradual increase in clastic mineral deposition (see Figure 7). Core images and smear slides following 2500 B.P. show an abundance of macro-plant fossils that occur suddenly in the Late-Holocene. The abrupt occurrence of macro-plant material is likely consistent with more shallow water and/or near shore taxa that contribute to the decline in the δ¹³C_OM discussed above. Increased Mn and decreased S suggest that this new shallow system is well oxygenated, as Mn is insoluble in oxygenated water and sulfide formation from decomposition requires anoxia. Notably, the increased organic matter is associated with a sharp decline in, and persistently low amounts of, biogenic silica around 2500 B.P. persisting nearly to the present day (Figure 7).

Conclusion

Chub Lake exhibits lake state changes that broadly coincide with continental scale changes in climate, emphasizing the utility of shallow lake records in climate reconstructions. Even further, high resolution Ca records from Chub Lake sediments records high-resolution variability in hydroclimate until 2500 B.P. that may correlate with millennial scale variability documented across the Northern Hemisphere at this time. Recent history in Chub Lake since 2500 B.P. to the present supports the importance of both lake basin geomorphology and climate in controlling hydrology, geochemistry, and ecology of shallow lake systems.

Supplemental Material

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Supplemental material, sj-csv-3-hol-10.1177_09596836241266426 for Shallow lake response to Holocene climate variation in south-central Minnesota, USA by E Rae Tennent, Daniel P Maxbauer and Charles E Umbanhowar in The Holocene

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Supplemental material, sj-R-5-hol-10.1177_09596836241266426 for Shallow lake response to Holocene climate variation in south-central Minnesota, USA by E Rae Tennent, Daniel P Maxbauer and Charles E Umbanhowar in The Holocene

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Supplemental material, sj-R-6-hol-10.1177_09596836241266426 for Shallow lake response to Holocene climate variation in south-central Minnesota, USA by E Rae Tennent, Daniel P Maxbauer and Charles E Umbanhowar in The Holocene

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Footnotes

Acknowledgements

We are grateful to the staff at the Continental Scientific Drilling (CSD) Facility at the University of Minnesota, particularly Kristina Brady, Mark Shapley, and Rob Brown, for their assistance with coring, data acquisition, and many helpful discussions. We would also like to thank Amy Myrbo, who provided guidance on site selection and coring and helped with early data collection. We also would like to thank St Olaf student Natalie Meinhardt for her assistance with quantifying charcoal abundance and Tom Brown for his assistance with dating.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was funded through the Boardman Fund and Allensworth Fund from the Carleton College Geology Department. Several Carleton students contributed to data collected in this study, in particular we thank Jordan Shapiro and all students from the GEOL 115 Climate Change in Geology winter 2019 course.

Data availability statement

The complete dataset reported here, as well as aerial photos, age model information, and statistical software used is available online as supplemental materials that accompany this paper.

ORCID iDs

Rae Tennent

Daniel P Maxbauer

Charles E Umbanhowar

Supplemental material

Supplemental material for this article is available online.

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