The palaeoproductivity of ancient Lake Superior

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Quaternary Science Reviews 30 (2011) 2988e3000

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Quaternary Science Reviews

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The palaeoproductivity of ancient Lake Superior

Ayumi Hyodo*, Fred J. Longstaffe**

The University of Western Ontario, Department of Earth Sciences, London, Ontario, Canada N6A 5B7

a r t i c l e i n f o

Article history:Received 28 December 2010Received in revised form20 June 2011Accepted 5 July 2011Available online 30 July 2011

Keywords:Lake SuperiorOrganic matterMeltwater terminationAlgaePrimary productivity

* Corresponding author. Present address: UniverDepartment of Geoscience, Weeks Hall, 1215 W DayUSA. Tel.: þ1 608 294 9476; fax: þ1 608 262 0693.** Corresponding author. Tel.: þ1 519 661 3177; fax

E-mail addresses: [email protected] (A. Hy(F.J. Longstaffe).

0277-3791/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.quascirev.2011.07.004

a b s t r a c t

We examine here variations in the organic carbon and nitrogen contents and isotopic compositions ofLake Superior sediments over the last 10,600 years, using bulk organic matter from four cores distributedacross the Lake Superior Basin. Very low Organic Carbon (OC) and Total Nitrogen (TN) contents but highCorg Mass Accumulation Rates (MARs) characterize these sediments until glacial meltwater supply to theBasin ended at w9000e8700 cal BP. The C/N ratios for organic matter from the glacial sediments spanthe range known both for organic matter adsorbed on soil clays and lacustrine algae grown underconditions of nitrogen deficiency. Organic matter in the glacial sediments has more-or-less uniformcarbon-isotope compositions (w�27&), perhaps associated with a steady flux of soil clays. Coexistingostracode valves vary by in carbon isotopic composition by as much as 5&, particularly between 10,600and 10,400 cal BP, reflecting changing DIC sources at this time.

In the postglacial sediments, the Corg MARs decreased sharply from glacial levels. The OC and TNconcentrations increased steadily until w6000 cal BP, after which OC and TN concentrations eitherstabilized or increased at a slower rate. The C/N ratios of the postglacial organic matter (mostly <10) aretypical of lacustrine algae. The carbon-isotope compositions of bulk organic matter in the postglacialsediments immediately after the end of glacial meltwater supply are lower (�29&) than those ofunderlying glacial sediments. An upward increase in the carbon-isotope compositions within the post-glacial sediments serves to characterize increasing primary productivity in the lake. This observation issupported by the small but steady rise in the nitrogen-isotope compositions of bulk organic matter in thepostglacial sediments towards present time.

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1. Introduction

Variations in aquatic primary productivity provide a good, first-order measure of a lacustrine system’s health during significantenvironmental change. The ancient lakes of the Lake Superior Basinhave been subject to such changes since their first appearance(w13,800 calibrated years [12.0 14C ka] BP) (Hough, 1958; Prest,1970; Saarnisto, 1975). During this time, the Lake Superior Basinexperienced several glacial advances and retreats, associatedchanges to its drainage systems and meltwater inputs from LakeAgassiz and the Laurentide Ice Sheet (LIS), and varying precipitationfluxes from the regional watershed, all arising from regional andglobal climatic fluctuations (Prest, 1970; Farrand and Drexler, 1985;

sity of WisconsineMadison,ton St., Madison, WI 53706,

: þ1 519 661 3198.odo), [email protected]

All rights reserved.

Breckenridge et al., 2004; Breckenridge, 2006, 2007; Breckenridgeand Johnson, 2009; Breckenridge et al., 2010).

Parts of the Lake Superior Basin were covered by the LIS untilw10,200 cal [9.1 14C ka] BP. The glacier and its meltwater deliveredsediments to the Basin from various sources, formed annual lami-nations (varves), and produced lake water compositions differentfrom the current condition (Breckenridge et al., 2004; Breckenridge,2007; Breckenridge and Johnson, 2009; Hyodo and Longstaffe,submitted for publication-a, submitted for publication-b). Basedon the oxygen-isotope compositions of ostracode valves, the LakeSuperior Basin was largely dominated by glacial meltwater (d18Oranged from �28 to �23&) until w9000e8700 cal BP when thissupply was terminated (Breckenridge and Johnson, 2009; Hyodo,2010; Hyodo and Longstaffe, submitted for publication-b). Shortlythereafter (within 400 calibrated years), the lake’s oxygen-isotopecomposition began to approach that of modern Lake Superiorbased on the oxygen-isotope composition of diatoms (Hyodo, 2010;Hyodo et al., 2010).

A stronger understanding of the response of ancient LakeSuperior to these climate-driven environmental shifts will allow us

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A. Hyodo, F.J. Longstaffe / Quaternary Science Reviews 30 (2011) 2988e3000 2989

to better anticipate the impacts of future climate modification inthe Great Lakes system. Changes in lake productivity are one suchresponse and can be tracked using lacustrine organic matter.Lacustrine organic matter reacts rapidly to changes in aquatic andterrestrial conditions, providing information concerning vegetationshifts and photosynthetic activity in a lake. Herewe seek to identifythe source(s) of organic matter in Lake Superior sediments, toevaluate changes in the activity of aquatic primary producers, andto understand the long-term climate-related changes in the lakeecosystem since w10,600 cal BP. We have measured the OrganicCarbon (OC) and Total Nitrogen (TN) contents and d13C and d15Nvalues of bulk organic matter from Lake Superior sediments anddetermined the d13C values of coexisting benthic ostracode valves.The ostracodes, which are present only in the glacial sediments,should record variations in the Dissolved Inorganic Carbon (DIC)composition of lake bottom water.

1.1. OC and TN contents and C/N ratios of lake sediments

OC and TN provide a measure of organic matter abundance insediments (Meyers and Teranes, 2001). Only less than 10% of theorganic carbon produced by photosynthesis at the water’s surfaceremains after passage through the water column (Eadie et al., 1984;Meyers and Eadie, 1993); nevertheless, relative changes in carbonand nitrogen contents generally remain related to changes in thelacustrine environment, including the flux of organic matter andthe supply rates of sediment. Organic matter in lacustrine sedi-ments is derived from terrestrial (vascular) plants and aquatic algae(non-vascular plants). Vascular plants contain large amounts ofcellulose (woody tissues) and waxy hydrocarbons, both of whichare rich in carbon. Non-vascular plants (phytoplankton) have lowcellulose and lignin contents, and are high in (N-rich) protein. Asa result, the atomic C/N ratio (atomic OC/TN ratio) can indicate thesource of organic matter in lake sediments: algal, C/N< 10 andterrestrial, C/N> 20 (Meyers and Ishiwatari, 1993a; Meyers, 1994,1997; Tyson, 1995; Meyers and Lallier-Verges, 1999). However,caution must be exercised when using this proxy; soilorganic matter adsorbed onto very fine-grained clays will typicallyhave C/N ratios <20 and in many cases <10 (e.g., Prahl et al., 1994;Batjes, 1996). Degradation can also modify the original C/N ratiosbut these changes are generally sufficiently small that terrestrialversus aquatic signals are preserved (Bourbonniere, 1979; Sollins

Fig. 1. Sediment core locations in the Lake Superior Basin. Core # 532, 18.35 m e Duluth suThunder Bay trough (48� 090 1800 N and 89� 010 3000 W; water depth, 285 m); Core # 534, 14.Core # 535, 18.24 m e Ile Parisienne sub-basin (46� 420 5400 N and 84� 470 1200 W; water d

et al., 1984; Ertel and Hedges, 1985; Talbot and Johannessen,1992; Meyers and Ishiwatari, 1993a).

1.2. Stable carbon- and nitrogen-isotope compositions of lakesediments

Most freshwater algae are C3 plants with carbon-isotopecompositions ranging from �29 to �25& (Meyers and Ishiwatari,1993a). Organic matter from terrestrial C3 plants has similar d13Cvalues (w�27&; Vogel, 1993). Terrestrial contributions of C4(w�13&) or CAM (w�20&) plants (Schidlowski et al., 1983; Fryand Sherr, 1984; O’Leary, 1988; Wolfe et al., 2001) increase thed13C values of organic matter in lacustrine sediments. However,such contributions are very unlikely during the Lake Superior Basinhistory considered here. Growth of C4 plants, in particular, is typi-cally associated with much higher minimum growing-seasontemperatures than experienced in this area at this time (Teeri andStowe, 1976; Woodward and Lomas, 2004; Dyke, 2005).

The d13C values of lacustrine algae are typically controlled bythe composition of dissolved CO2 (DIC) and the level of photosyn-thetic activity (McKenzie, 1985; O’Leary, 1988; Wolfe et al., 1996,2001; Hodell and Schelske, 1998). When organic matter in sedi-ments is derived mostly from such algae, its isotopic compositioncan be used to describe cycling of DIC as well as primary aquaticproductivity. DIC d13C values vary with the composition of atmo-spheric CO2, the extent of exchange between atmospheric anddissolved CO2, the soil CO2 flux into the lake (Rau, 1978; Boutton,1991; Wolfe et al., 1996, 1999), dissolution of carbonate detritus(Meyers, 2003), and the decay rates of organic matter in the sedi-ments (Hayes, 1993). In low-organic sediments (OC less than a fewpercent), the original d13C values of organic matter typicallyundergo little change e even after microbial alteration (Rea et al.,1980; Jasper and Gagosian, 1989; Meyers and Ishiwatari, 1993b;Hodell and Schelske, 1998; Meyers and Lallier-Verges, 1999).

Algal productivity depends on several factors, including theavailability of DIC and nutrients in lake water (e.g. phosphate,nitrate, silicic acid), water temperature, and light intensity (Deuserand Degens, 1967; Rhee and Gotham, 1981; Ingraham and Taylor,1991; Indermuhle et al., 1999). While phytoplankton prefer toutilize 12C during photosynthesis, increased productivity reducesthis discrimination, causing d13C values to increase (Herczeg, 1988;Farquhar et al., 1989; Hollander and McKenzie, 1991; Lücke andBrauer, 2004).

b-basin (47� 110 0000 N and 91� 130 4200 W; water depth, 279 m); Core # 533, 17.57 m e

69 m e Caribou sub-basin (47� 320 3000 N and 87� 000 0000 W; water depth, 313 m), andepth, 123 m).

A. Hyodo, F.J. Longstaffe / Quaternary Science Reviews 30 (2011) 2988e30002990

The nitrogen source for lacustrine, non-nitrogen-fixing algae isDissolved Inorganic Nitrogen (DIN; e.g., NO3

�, NO2�, NH4

þ) derivedfrom soil and/or nitrogen fixed microbially in lake water (Goerickeet al., 1994). The d15N values of these algae typically range from þ2to þ13& (e.g., Nichols and Garling, 2000; Johannsson et al., 2001;Vuorio et al., 2006), depending on the nitrogen isotopic composi-tion of the DIN reservoir and algal productivity (Meyers, 2003). Bycomparison, land plants utilize inorganic nitrogen (NO3, NO2, NH4)available in soil, derived mainly through nitrification of microbiallyfixed atmospheric N2 (Fogel and Cifuentes,1993). The d15N values ofmost land plants are typically much lower (�5 to þ3&) thanorganic matter of lacustrine origin, though higher values for landplants are known from some situations, and a tendency forenrichment in 15N to accompany lower mean annual precipitationand higher mean annual temperature has been reported (Ambrose,1991; Fry, 1991; Kendall, 1998; Amundson et al., 2003).

The d15N values of the DIN reservoir are affected by denitrifi-cation (Cline and Kaplan, 1975; Teranes and Bernasconi, 2000),input from agricultural runoff and sewage (Peterson and Fry, 1987;Hodell and Schelske, 1998), leaching of soil-derived nitrate (Peterset al., 1978), abundance of N-fixing bacteria (Fogel and Cifuentes,1993), and ammonia volatilization (Talbot and Lærdal, 2000).When primary algal productivity is high, a higher proportion of theDIN reservoir is consumed, which increases the d15N values of theremaining reservoir, thus producing an increase in the d15N valuesof subsequently produced organic matter (Wada and Hattori, 1991;Fogel and Cifuentes, 1993; Sigman et al., 1997; Kendall, 1998; Talbotand Lærdal, 2000).

Preferential release of 14N from organic matter can occur duringits passage through the water column and subsequent burial

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gV

Fig. 2. Age-depth models and brief sediment descriptions for the four sediment cores exrhythmites (r/gR), red rhythmites (rR), brown clay (bC), grey clay (gC), red clay (rG). CloRadiocarbon Accelerator Unit). *The sedimentation rates used for the postglacial sediments aa) for more details.

(Cifuentes et al., 1989; Altabet et al., 1991; Meyers and Ishiwatari,1993a). In Lake Michigan, however, a range of less than 2& in thed15N values of organic matter was observed at different waterdepths (Meyers and Eadie, 1993). We posit that trends in the d15Nvalues of lacustrine organic matter entrapped in sediments remaingood indicators of relative changes in the composition of the DINreservoir and/or algal productivity over longer periods of time.

2. Materials and methods

In 2005, five-centimetre-diameter piston cores were collectedfrom four sub-basins within Lake Superior (Fig. 1): Duluth sub-basin (47� 110 0000 N and 91� 130 4200 W; water depth, 279 m) e

Core # 532, 18.35 m; Thunder Bay trough (48� 090 1800 N and 89� 010

3000 W; water depth, 285 m) e Core # 533, 17.57 m; Caribou sub-basin (47� 320 3000 N and 87� 000 0000 W; water depth, 313 m) e

Core # 534,14.69 m, and Ile Parisienne sub-basin (46� 420 5400 N and84� 470 1200 W; water depth, 123 m) e Core # 535, 18.24 m. Thecores were stored at 4 �C prior to analysis. Sediment recoveredbetween w18 and w14e15 m in Cores # 532, 533 and 534 consistsof homogeneous, massive clay; its occurrence is anomalous relativeto the known stratigraphy of the area (e.g., Breckenridge, 2007).Porewater from these intervals also has a significantly differentisotopic profile than overlying sediment (Hyodo, 2010). We haveconcluded that this sediment was drawn into the bottom of thepiston corer during its recovery. This material has been excludedfrom further discussion. Core descriptions are provided in Hyodoand Longstaffe (submitted for publication-a). Age-depth modelsfor these cores (Fig. 2, Hyodo and Longstaffe, submitted forpublication-a) are based upon six AMS radiocarbon dates

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8,854 (9,124-8,595) cal BP8,094 (8,295-7,954) cal BP

gVPC*

8,869 (9,397-8,430) cal BP8,629 (8,951-8,482) cal BP

bCgC

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amined here: postglacial clay (PC), grey varves (gV), grey rhythmites (gR), red/greysed circles are radiocarbon dates (and 2 range) calibrated using Oxcal 4.1 (Oxfordre taken from Kemp et al. (1978). See Hyodo and Longstaffe (submitted for publication-

Fig. 3. a. Calibrated years before AD 1950 (cal BP) versus Organic Carbon (OC) for the Lake Superior Basin cores. The dashed lines represent 9300, 8800, 8200, 6000 and 3000 cal BP,respectively. The dashed line at 8800 cal BP approximates the glacialepostglacial boundary. b. Mass Accumulation Rates (MARs) for Organic Carbon (OC) calculated using linearsedimentation rates, dry density of sediments, and OC. c. Cal BP versus Total Nitrogen (TN). The dashed lines represent 9300, 8800, 8200, 6000 and 3000 cal BP, respectively. Thedashed line at 8800 cal BP approximates the glacialepostglacial boundary. d. Mass Accumulation Rates (MARs) for Total Nitrogen (TN) calculated using linear sedimentation rates,dry density of sediments, and TN contents.


Fig. 3. (continued).



obtained for rare macrofossils of terrestrial organic matter, varvecounts, and correlation with varve sequences reported previouslyfor Lake Superior sediments (Breckenridge et al., 2004).

2.1. Bulk organic matter analyses

Three-centimetre-thick samples were freeze-dried and groundby hand in a ceramic mortar using a rubber pestle. Prior tomeasurement of OC and TN concentrations, and carbon-isotopecompositions, carbonates were removed from the samples usinga method modified from Verardo et al. (1990). Briefly, for carbonisotopic analysis, the sediments were reacted with 1.0 ml ofsulphurous acid (6.4% SO2) for 2 h and heated at 60 �C to decom-pose the sulphurous acid into SO2 gas and water. This process wasrepeated with larger quantities of sulfurous acid (10 ml, 20 ml,40 ml) until no bubbles were observed. Untreated samples wereused for measurement of nitrogen-isotope compositions in order toavoid changes in d15N values resulting from acid treatment(Verardo et al., 1990).

Thirty milligrams of the dry sediments were weighed into tincups, and their OC and TN concentrations measured using a Fisons1108 Elemental Analyzer. The concentrations were calibrated usingthe following standards: Low Organic Content Soil (OC 1.52 wt.%,TN 0.13 wt.%) and High Organic Content Soil (OC 6.10 wt.%, TN0.46 wt.%). The precision of duplicate analyses for samples (n¼ 13)was normally better than �0.07 wt.% for OC and �0.01 wt.% for TN(1 ). These results were used to calculate atomic C/N ratios (Meyers,1994), which were reproducible to �0.4 (1 ).

A Costech Elemental Combustion System connected to a ThermoFinnigan DeltaPLUS XL mass spectrometer in continuous-flow (He)mode was used to determine the isotopic compositions of theorganic matter relative to Vienna Pee Dee Belemnite (VPDB) forcarbon (Coplen, 1996) and air for nitrogen (Hoefs, 1997). Calibrationcurves were derived using the following standards: IAEA-CH-6(�10.5&) and NBS-22 (�30.0&) for carbon, and IAEA-N1 (0.4&)and IAEA-N2 (20.3&) for nitrogen. Keratin was analyzed as aninternal standard to determine the accuracy and precision of theisotopic analysis. The average d13C value of keratinwas �24.0� 0.1& (1 , n¼ 32), and the average d15N value was6.3� 0.1& (1 , n¼ 6), which compares well with its accepted valuesof�24.1 and 6.4&, respectively. Sample reproducibility based on 12replicates was �0.2& for carbon and �0.1& for nitrogen. The lowTN contents of glacial sediments (Duluth sub-basin, 5.50e18.35 m;Thunder Bay trough, 7.50e18.37 m; Caribou sub-basin,3.50e18.19 m; Ile Parisienne sub-basin, 14.37e18.24 m) precludedmeasurement of their nitrogen-isotope compositions.

2.2. Ostracode analyses

Sediments were sampled at 10 cm intervals, and wet-sievedusing 500, 250 and 125 mm mesh-sizes. Ostracode valves werepicked from the dried sieves, and any valves with adhered mudwere cleaned using distilled water and a camel-hair brush, andthen dried at room temperature. No evidence for chemical orphysical alteration of the valve surfaces was observed microscopi-cally (Hyodo, 2010). All adult valves were identified as Candonasubtriangulata.

The stable carbon-isotope compositions of the ostracodes wereobtained by reaction with orthophosphoric acid at 90 �C usinga Micromass MultiPrep device coupled to a VG Optima massspectrometer in dual-inlet mode. The carbon-isotope ratios areexpressed in per mil (&) using the standard delta notation (d)relative to VPDB (Coplen, 1996). The calibration produced acceptedvalues for standards, including NBS-19 (1.95&) and Suprapur(�35.3&). Standards WS-1 and NBS-18 were used to evaluate the

accuracy and precision of the analyses. For WS-1, the average d13Cvalue was 0.8� 0.1& (1s, n¼ 30), and for NBS-18, �5.0� 0.1& (1s,n¼ 58), both results being identical to the accepted values for thesestandards. The precision of duplicate analyses of samples wasnormally better than �0.2& (1s, n¼ 20).

3. Results and discussion

3.1. Organic Carbon, Total Nitrogen and C/N ratios

The OC abundances measured for these Lake Superior sedimentcores are very low (<0.5%) prior to 9000e8700 cal BP, a periodduring which the Lake Superior Basin was still receiving glacialmeltwater from the LIS (Fig. 3a, see Supplementary material).Following final termination of the meltwater supply, however, theamounts of OC increase to w1.5% between 9000e8700 and6000 cal BP, and then rise more gradually to w2% between6000 cal BP and near-present time. The termination of meltwatersupply was marked by secession of varve formation (Breckenridgeet al. 2004; Hyodo and Longstaffe, submitted for publication-a),and by a sharp increase in the d18O values of ancient lake waterfrom w�28 to w�15& (VSMOW), as inferred from the oxygenisotopic compositions of ostracodes and diatoms from LakeSuperior glacial and postglacial sediments (Hyodo, 2010; Hyodoet al., 2010).

Changes in OC abundances can reflect shifts in primaryproductivity, but they can also arise from variations in sedimenta-tion rates. Mass Accumulation Rates (MARs) are used to assess thispossibility (Meyers and Takemura, 1997; Meyers, 2003; Lücke andBrauer, 2004). We have calculated Corg MARs (mg/cm2/year) forour samples following Meyers and Takemura (1997) and Yu et al.(2007), where Corg MARs¼OC� LSR�DBD� 1000, and OC isOrganic Carbon content (wt.%), LSR is linear sedimentation rate(cm/year), and DBD (g/cm3) is dry bulk density determined by thedry weight per unit wet volume. Our results indicate periods ofvery high Corg MARs while glacial meltwater flowed into the LakeSuperior Basin for all but the Ile Parisienne sub-basin (Fig. 3b). Suchvalues may be suggestive of high aquatic primary productivity atthis time but alternatively could indicate a significant terrestrialorganic input delivered by meltwater.

The TN abundances show the same trends as OC (Fig. 3c; seeSupplementary material): low values before w9000e8700 cal BP(<0.1%) followed by an increase to w0.2% between w9000e8700and w6000 cal BP, and a more gradual increase to w0.3% betweenw6000 cal BP and near-present time. When these amounts arerecast as nitrogen mass accumulation rates (Norg MARs) (Fig. 3d),they demonstrate the same patterns as described for Corg MARs.

The TN values are generally used as a proxy of total organicnitrogen concentration in sediments because the amount of inor-ganic nitrogen is normally low compared to organic nitrogen(Meyers, 1997). In low-organic sediments (OC< 0.3), however,inorganic nitrogen (e.g. ammonia potentially associated with clayminerals) can comprise a significant fraction of TN and serve tolower C/N ratios (Rau et al., 1987; Calvert, 2004). We have tested forthis possibility using a cross-plot of TN versus OC (Fig. 4). Theintercept of the TNeOC regression on the TN axis is a measure ofsediment inorganic nitrogen content (Nijenhuis and de Lange,2000), provided that it is constant across the system (Calvert,2004). Using this approach, very low inorganic nitrogen contents(<0.01%) are indicated for the samples examined here. Hence, theTN, Norg MARs and d15N values reported here are interpreted asthose of organic nitrogen.

The C/N ratios of the Lake Superior organic matter range from 7to 17 prior to 9000e8700 cal BP, except for the Ile Parisienne sub-basin where the C/N ratios are, with one exception (10.9) <10

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Fig. 4. Total Nitrogen (TN) versus Organic Carbon (OC) contents. The intercept on the TN axis is considered to represent Total Inorganic Nitrogen, as determined using a Model 2regression (Ricker 1984).


(Fig. 5) (see Supplementary material). These ratios span a rangehigher than typical for lacustrine aquatic vegetation (<10) butlower than typical for terrestrial vegetation (>20). Phytoplanktonformed under conditions of nitrogen deficiency can have C/N ratiosof 13e20 (Hecky et al., 1993; Talbot and Lærdal, 2000). Hence it ispossible that this organic matter derives from algae grown undernutrient-deficient conditions. Such an origin, however, would beinconsistent with the flux of organic matter suggested by the Corgand Norg MARs; such productivity seems very unlikely during thisglacial period.

A discrete terrestrial origin for this organic matter also seemsunlikely. Terrestrial organic input is generally considered to havebeen very small in the Great Lakes (Halfon, 1984; Meyers andIshiwatari, 1993a; Ostrom et al., 1998) and the LIS covered thenorthern Great Lakes region during this time (Leverington et al.,2000, 2002). Indeed, only 13 small organic macrofossils werediscovered in >75 m of core examined here (Hyodo, 2010).However, a possible source with suitable C/N ratios is soil organicmatter adsorbed onto clays that had been transported into theBasin by glacial meltwater. Indeed, clayminerals comprise 20�10%of the glacial sediment in each core examined, except for the IleParisienne sub-basin (10� 5%) (Hyodo and Longstaffe, submittedfor publication-a). Additional biogeochemical tests are needed toevaluate this possibility.

After 9000e8700 cal BP, OC and TN abundances graduallyincrease and the Corg and Norg MARs stabilize at much lower levelsthan determined for the glacial sediments (Fig. 3aed). The C/Nratios of the organic matter (mostlyw7e9) are typical of lacustrinealgae produced under normal nutrient conditions (Fig. 5). Wesuggest that the higher OC and TN concentrations post-w8800 cal BP largely reflect increasing primary (algal) productivityin the Lake Superior Basin.

3.2. Carbon-isotope composition of bulk organic matter

Prior to w8800 cal BP, the d13COM values in the Duluth andCaribou sub-basins, and the Thunder Bay trough fluctuate slightlyabout an average of �27.2& (�0.3&, 1 ) (Fig. 6a; seeSupplementary material). While such compositions are consistentwith those of terrestrial soil organic matter adsorbed on clays, anorigin as lacustrine algal organic matter cannot be ruled out basedon these isotopic data alone.

A larger fluctuation (�1.3&, 1 ) about a slightly higher averagevalue (�26.5&) for d13COM occurs in the Ile Parisienne sub-basin(Fig. 6a). Assuming a terrestrial origin, this difference in organicmatter carbon isotopic composition and variability relative to theother cores conceivably could arise from clay particle size-fractionation. The mean grain-size of Ile Parisienne sediments inthis interval is 1.8 mm, compared to <0.5 mm for the locations,which were sampled from much deeper water (Hyodo andLongstaffe, submitted for publication-a). However, the muchlower Corg and Norg MARs and lower clay mineral contents relativeto the other cores, coupled with C/N ratios <10, present a morecompelling argument that these d13Corg values reflect fluctuatinglacustrine productivity and/or changes in the cycling and compo-sition of DIC in the Ile Parisienne sub-basin at this time (perhaps inpart arising from a freshwater flood/outburst from Lake Superior inthis region at w9300 cal BP; Yu et al., 2010).

Between w9200 and w7700e7600 cal BP, the d13COM valuesdecreased to w�29&, first in the Ile Parisienne sub-basin(w9,200 cal BP; 15.66 m), then in the Thunder Bay trough(w8400 cal BP, 7.50 m), and finally in the Duluth sub-basin(w8000e7600 cal BP, 4.80e4.50 m) (Fig. 6a). Given the shift in Corgand Norg MARs and C/N ratios described earlier, we suggest thatthese compositions represent an initial baseline for postglacial

Fig. 5. cal BP versus C/N ratios. The dashed lines represent 9300, 8800, 8200, 6000 and 3000 cal BP, respectively. The dashed line at 8800 cal BP approximates the glacialepostglacialboundary.


lacustrine algal productivity. Such d13COM values would be expectedfor conditions of low algal productivity and/or abundant dissolvedCO2, when discrimination against 13CO2 during algal uptake isenhanced (Herczeg, 1988; Farquhar et al., 1989; Hollander andMcKenzie, 1991; Fogel and Cifuentes, 1993; Street-Perrott et al.,1997; Hollander and Smith, 2001; Lücke et al., 2003; Lücke andBrauer, 2004). While the occurrence of abundant dissolved CO2 atthis time cannot be ruled out, we note that the supply of dissolvedinorganic carbon decreased dramatically in the Lake Superior Basinwith the end of glacial meltwater supply (Dell, 1973; Kemp et al.,1978; Breckenridge, 2006). Glacial meltwater inflow into theLake Superior Basin likely contained abundant DIC from dissolutionof Palaeozoic carbonate detritus to the north (e.g., Lake Agassizregion, Hudson Bay lowlands). During postglacial sedimentation,a much more limited flux of carbonate detritus would have origi-nated from more proximal locations within the Lake Superiorwatershed; this carbonate may have had different d13C values thanthat delivered by glacial meltwater (Hyodo and Longstaffe,submitted for publication-a).

Between w8000 and w3000 cal BP, the d13COM valuesincreased from w�29& across the Lake Superior Basin to �27.5&in the Duluth sub-basin, �27& in the Thunder Bay trough,and �24& in the Ile Parisienne sub-basin (Fig. 6a). This increaselikely reflects growing primary productivity, as also indicated bythe increase in OC and TN contents. The Lake Superior Basinwatershed became forested following the retreat of the LIS and asregional temperatures rose, and as a result, we anticipate thatgreater quantities of nutrients were delivered to the lake. Theappearance of diatom blooms at this time is consistent withincreased nutrient supply to postglacial Lake Superior (Hyodo,

2010). The resulting increase in productivity would have reduceddiscrimination against 13CO2 during algal photosynthesis (Herczeg,1988; Farquhar et al., 1989; Hollander and McKenzie, 1991; Lückeand Brauer, 2004).

During the postglacial sedimentation, organic matter in the IleParisienne sub-basin attained d13COM values up to 2e3& higherthan in the Duluth sub-basin or Thunder Bay trough at anyparticular time (Fig. 6a). This increase is not likely related to directterrestrial input, given that C/N ratios are similar in all three loca-tions (<10) (Fig. 5). However, the Ile Parisienne sub-basin is situ-ated much closer to present and past shorelines, and water depth(presently 123 m) was likely shallower than in the other areas(present depths >200 m), which may have facilitated higher algalproductivity.

3.3. Nitrogen-isotope composition of bulk organic matter

The low TN contents of samples older than w9000e8700 cal BPprecluded measurement of their nitrogen-isotope compositions.For materials younger than w9000e8700 cal BP, the d15NOMvalues generally increased upwards in the cores: Duluth sub-basin, þ3 to þ4&; Thunder Bay trough, þ3 to þ5&; and Ile Par-isienne sub-basin, þ2 to þ4& (Fig. 6b) (see Supplementarymaterial). Such compositions trend towards the d15NOM values(wþ5&) of Lake Superior surface sediments (0e50 cm depth)(Pang and Nriagu, 1977). This increase is consistent with increasedaquatic primary productivity. When aquatic primary productivity ishigh, utilization of dissolved inorganic nitrogen increases, reducingalgal discrimination against 15N (Meyers, 2003). Denitrification alsocould have contributed to this 15N-enrichment. During

Fig. 6. a. cal BP versus d13COM between 10,600 cal BP and present. The dashed lines represent 9300, 8800, 8200, 6000 and 3000 cal BP, respectively. The dashed line at 8800 cal BPapproximates the glacialepostglacial boundary. b. cal BP versus d15NOM. The dashed lines represent 8800, 6000 and 3000 cal BP, respectively. The dashed line at 8800 cal BPapproximates the glacialepostglacial boundary.



denitrification, 14NO3� is selectively utilized by bacteria, leaving 15N-

enriched nitrate behind in solution (Arthur et al., 1983; Collister andHayes, 1991). This nitrate is then consumed during algal photo-synthesis, leading to higher d15NOM values (Teranes and Bernasconi,2000).

3.4. Carbon-isotope composition of ostracode valves

The d13Costracode values show little systematic variation (<3&)throughout the ostracode-bearing (w10,600 tow9000e8700 cal BP)sediments of the Caribou and Ile Parisienne sub-basins and theThunder Bay trough (excepting one outlier of �1.4&) (Fig. 7) (seeSupplementary material). The fluctuations lie within the rangepossible fromvarying incorporation ofmetabolic carbon and variableDIC compositions induced by ostracodes within their micro-environments (Dettman et al., 1995). Larger and more systematicchanges are observed in the Duluth sub-basin, where d13Costracodevalues range from�7 to�2& betweenw10,600 andw10,400 cal BP(Figs. 7 and 8). Red sediments containing allochthonous calcite withd13C values of�4.2 to�2.6&were deposited in the Duluth sub-basinprior tow10,500 cal BPwhereas the allochthonous calcite has higherd13C values (�1.1 to �0.7&) in the overlying grey sediments (Hyodoand Longstaffe, submitted for publication-a). Dissolution of thesedifferent varieties of calcite could largely explain thewider spread ind13Costracode values during this period of time.We also note the small(w1.5&) but systematic increase in d13COM values that accompaniesthe red to grey clay transition from w10,600 to w10,400 cal BP(Fig. 8). This could arise in several ways, including, but not limited to,mixing of different sources of clays carrying adsorbed organicmatter(withdifferent d13COMvalues), as has beenposited for the carbonates,or progressive dilution of lacustrine organic matter by organic-

Fig. 7. cal BP versus the carbon-isotope composition of ostracode valves. The dashed lines repthe glacialepostglacial boundary.

bearing clays. Until the ambiguity surrounding the source of theorganic matter in the glacial sediments is resolved, further commentis not warranted.

3.5. Productivity and environmental change in the Lake SuperiorBasin

Primary productivity in ancient Lake Superior began to increaseonce glacial meltwater input ended. Subsequent climatic pertur-bations (‘events’) appear not to have interrupted this pattern ofmodestly increasing productivity. For example, hemispheric-scalecooling events apparently accompanied the collapse of the LISand accompanying inflow of freshwater into Hudson Bay atw8200 cal BP (Dredge, 1983; Teller et al., 2002; Clarke et al., 2004)but a consistent signal of a short-term shift to colder conditions isnot apparent in the data reported here for that time. Likewise, atw5000 cal BP, climatic conditions in the Great Lakes regionchanged from dry andwarm towet andwarm (Edwards et al., 1996;Lewis et al., 2008), and at w3000 cal BP, regional cooling is sug-gested by a lowering of Lake Superior d18O valuese as inferred fromdiatom oxygen isotopic compositions (Hyodo, 2010). Yet noconsistent patterns in the trends for OC and TN contents, Corg andNorg MARs, or d13COM and d15N values can be immediately associ-ated with those times. The large volume and area of Lake Superiormay have muted productivity changes related to these events.Smaller Great Lakes are likelymore sensitive to such environmentalshifts (Silliman et al., 1996).

Only a few previous studies have examined the organic matterisotopic composition of the Great Lakes over the lastw11,000 cal BP(e.g., Lake Huron: Meyers et al., 1980; Lake Ontario: Silliman et al.,1996). More detailed studies of all Great Lakes ewithin the context

resent 9300 and 8800 cal BP, respectively. The dashed line at 8800 cal BP approximates

- 28 - 27 - 8 - 7 - 6 - 5 - 4 - 3 - 2 - 1 0

13 C OM (‰, VPDB) 13 C (‰, VPDB)

8600

8900

28 278600

8900

8 7 6 5 4 3 2 1 0

9200 9200 Bulk calcite

9500 9500 Grey clay

Cal

BP

Cal

BP

9800

10100

9800

10100 Bulk organic

Ostracode

10400 10400

Bulk organicmatter

Red/grey clay

10700 10700 Red clay

Fig. 8. cal BP versus the d13C values of bulk organic matter (filled circles), ostracode valves (open circles) and bulk calcite (crosses) from the Duluth basin.


of well-constrained age models e are needed to fathom fully howproductivity in this enormous freshwater reservoir has respondedto environmental and climatic change in the past.

4. Conclusions

Glacial sediments (w10,600 to 9000e8700 cal BP) from theDuluth and Caribou sub-basins and Thunder Bay trough of the LakeSuperior Basin are characterized by low OC and TN contents, vari-able but oftentimes high Corg and Norg MARs, and relativelyconstant d13COM values (w�27&). The origin of the organic matterin the glacial sediments is uncertain. Its C/N ratios (7e17) aretypical both of lacustrine algae grown under nitrogen deficientconditions and terrestrial organic matter adsorbed onto soil clayminerals. Presently, we favour the second suggestion. Variations inthe d13C values of ostracode valves, particularly in the Duluth sub-basin between w10,600 and w10,400 cal BP, most probably reflecta change in source for the allochthonous carbonate depositedduring the transition from red to grey clay glacial sedimentation.

A record of Lake Superior’s low but increasing primary produc-tivity is preserved in postglacial sediments from the Duluth sub-basin and Thunder Bay trough, and the entire sedimentary recordrecovered for the Ile Parisienne sub-basin. At w9000e8700 cal BP,initial d13COM and d15NOM values were w�29& and wþ2&,respectively, Corg MARs andNorgMARs had stabilized at levels muchlower than in glacial times, and C/N ratios (7e9) were typical oflacustrine algae. From this baseline, increasing productivity ismarked by a systematic increase in OC and TN contents, and d13COMand d15NOM values towards present time.

Acknowledgements

This manuscript has been improved substantially by theperceptive ideas and comments of two anonymous reviewers, andthrough the efforts of the journal’s editorial team. In addition, wethank Allan Crowe (Canada Centre for Inland Waters) and theCaptain and crew of the CCGC Limnos for facilitating core collection,and the staff of the Laboratory for Stable Isotope Science (TheUniversity of Western Ontario) for assistance with the stableisotopic analysis. The Natural Sciences and Engineering Research

Council of Canada and the Canada Foundation for Innovationprovided financial support. This is Laboratory for Stable IsotopeScience Contribution # 265.

Appendix. Supplementary material

Supplementary data associated with this article can be found inthe online version, at doi:10.1016/j.quascirev.2011.07.004.

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The palaeoproductivity of ancient Lake Superior

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