Jane L. Teranes

Sediment Organic Matter

The organic matter content of lake sediments provides a variety of indicators, or proxies, that can be used to reconstruct paleoenvironments of lakes and their watersheds and to infer histories of regional climate changes. Organic matter constitutes a minor but important fraction of lake sediments. It originates from the complex mixture of lipids, carbohydrates, proteins, and other organic matter components produced by organisms that have lived in and around the lake (e.g., Meyers, 1997; Rullkotter, 2000). As an accumulation of “geochemical fossils”, the organic matter content of lake sediments provides information that is important to interpretations of both natural and human-induced changes in local and regional ecosystems.

Varve formation since 1885 and high-resolution varve analyses in hypertrophic Baldeggersee (Switzerland)

According to their microstratigraphical composition the laminated sediments of Baldeggersee are true varves. Two varve time-series from freeze cores taken in the deepest part (66 m) of Baldeggersee have been analysed for annual and seasonal layers. The varve time-series covers the period of 1885 to 1993.The two freeze-cores from Baldeggersee can be accurately correlated by means of distinct layers (e.g. marker varves, turbidites), lithological units, and varve measurements. The Baldeggersee varve chronology has been checked by independent high-resolution137Cs dating. Mobility of cesium has not been detected in Baldeggersee. A highly positive correlation between phosphorus concentration in the lake water and median grain size of calcite crystals has been observed and allows, together with the results of geochemical analyses, to hindcast the trophic state in Baldeggersee for periods where no limnological data are available.The main features of the Baldeggersee varve time-series can be summarized as follows: before 1885 there are packets of 5–10 varves interrupted by massive, homogeneous marl beds, indicating oscillating hypolimnetic oxygen levels until the lake ultimately became anoxic in 1885. Between 1885 and 1905 varves are thin, with a high carbonate content. Between 1905 and 1910 the thickness of varves increased steadily, and an increase in calcite grain size suggests a major step in eutrophication. Median grain size values increase again at the end of the 1940s, whereas the thickness of the seasonal layers decreased between the early 1940s and the early 1960s. At the onset of the 1960s, during the time of highest epilimnetic phosphorus concentrations in Baldeggersee, the total phosphorus concentrations in the sediments as well as the varve thickness and the total accumulation rates all increased. At the beginning of the 1980s the median grain size decreased substantially as a result of lower phosphorus concentrations in Baldeggersee.

Graphite and carbonates in the 3.8 Ga old Isua Supracrustal Belt, southern West Greenland

We present a systematic study of abundance, isotopic composition and petrographic associations of graphite in rocks from the ca. 3.8 Ga Isua Supracrustal Belt (ISB) in southern West Greenland. Most of the graphite in the ISB occurs in carbonate-rich metasomatic rocks (metacarbonates) while sedimentary units, including banded iron formations (BIFs) and metacherts, have exceedingly low graphite concentrations. Regardless of isotopic composition of graphite in metacarbonate rocks, their secondary origin disqualifies them from providing evidence for traces of life stemming from 3.8 Ga. Recognition of the secondary origin of Isua metacarbonates thus calls for reevaluation of earlier interpretations that suggested the occurrence of 3.8 Ga biogenic graphite in these rocks. Thermal decomposition of siderite; 6FeCO3 = 2Fe3O4 + 5CO2 + C, is the process seemingly responsible for the graphite formation. The cation composition (Fe, Mg, Mn, and Ca) of the carbonate minerals, carbon isotope ratios of carbonates and associated graphite and petrographic assemblages of a suite of metacarbonates support the conclusion that multiple pulses of metasomatism affected the ISB, causing the deposition of Fe-bearing carbonates and subsequent partial disproportionation to graphite and magnetite. Equilibrium isotope fractionation between carbonate and graphite in the rocks indicates peak metamorphic temperatures between 500 and 600 ◦ C, in agreement with other estimates of metamorphic temperature for the ISB.

Prehistorical record of cultural eutrophication from Crawford Lake, Canada

Cultural eutrophication—the process by which human activities increase nutrient input rates to aquatic ecosystems and thereby cause undesirable changes in surface-water quality—is generally thought to have begun with the start of the industrial era. The prehistoric dimension of human impacts on aquatic ecosystems remains relatively undescribed, particularly in North America. Here we present fossil plankton data (diatoms and rotifers), organic and inorganic carbon accumulations, and carbon isotope ratios from a 1000 yr sediment core record from Crawford Lake, Ontario, Canada. The data document increased nutrient input to Crawford Lake caused by Iroquoian horticultural activity from A.D. 1268 to 1486 and show how this increased nutrient input elevated lake productivity, caused bottom-water anoxia, and irreversibly altered diatom community structure within just a few years. Iroquoian settlement in the region declined in the fifteenth century, yet diatom communities and lake circulation never recovered to the predisturbance state. A second phase of cultural eutrophication starting in A.D. 1867, initiated by Canadian agricultural disturbance, increased lake productivity but had comparatively less impact on diatom assemblages and carbon-storage pathways than the initial Iroquoian disturbance. This study deepens our understanding of the impact of cultural eutrophication on lake systems, highlights the lasting influence of initial environmental perturbation, and contributes to the debate on the ecological impacts of density and agricultural practices of native North American inhabitants.

A study of oxygen isotopic fractionation during bio-induced calcite precipitation in eutrophic Baldeggersee, Switzerland

In order to better understand environmental factors controlling oxygen isotope shifts in autochthonous lacustrine carbonate sequences, we undertook an extensive one-year study (March, 1995 to February, 1996) of water-column chemistry and daily sediment trap material from a small lake in Central Switzerland. Comparisons between calculated equilibrium isotope values, using the fractionation equation of Friedman and O’Neil, (1977) and measured oxygen isotope ratios of calcite in the sediment-traps reveal that oxygen isotopic values of autochthonous calcite (δ18O) are in isotopic equilibrium with ambient water during most of the spring and summer, when the majority of the calcite precipitates. In contrast, small amounts of calcite precipitated in early-spring and again in late-autumn are isotopically depleted in 18O relative to the calculated equilibrium values, by as much as 0.8‰. This seasonally occurring apparent isotopic nonequilibrium is associated with times of high phosphorous concentrations, elevated pH (∼8.6) and increased [CO32−] (∼50 μmol/l) in the surface waters. The resulting weighted average δ18O value for the studied period is −9.6‰, compared with a calculated equilibrium δ18O value of −9.4‰. These data convincingly demonstrate that δ18O of calcite are, for the most part, a very reliable proxy for temperature and δ18O of the water.

Lacustrine oxygen isotope record of 20th-century climate change in central EuropeL evaluation of climatic controls on oxygen isotopes in precipitation

We report oxygen isotope data from a 108-yr (1885–1993) sequence with annual laminae of bio-induced authigenic calcite in a frozen core from Baldeggersee, a small lake in Central Switzerland. These isotope results provide proxy data on the isotopic composition of past precipitation in the Baldeggersee catchment region and are quantitatively compared with instrumental climate data (i.e. mean annual air temperature and atmospheric circulation pattern indices) to evaluate climatic controls on oxygen isotopes in precipitation.Monitoring the isotope hydrology of Baldeggersee demonstrates that the oxygen isotopic composition of the lake water is controlled by the isotopic composition of local atmospheric precipitation (δ18Op) and that the isotopic signal of precipitation is preserved, albeit damped, in the lake calcite oxygen isotope record (δ18Oc). Comparison of the calcite oxygen isotope proxy for δ18Op in the catchment with historical mean annual air temperature measurements from Bern, Switzerland confirms that authigenic calcite reliably records past annual air temperature in the region. This δ18Oc/temperature relationship is calculated to be 0.39‰/°C for the period 1900–1960, based on an isotope mass-balance model for Baldeggersee. An exception is a 0.8‰ anomalous negative shift in calcite δ18O values since the 1960s. Possible explanations for this recent δ18Oc shift, as it is not related to mean annual air temperature, include changes in δ18Op due to synoptic circulation patterns. In particular, the 0.8‰ negative shift coincides with a trend towards a more dominant North Atlantic Oscillation (NAO) index. This circulation pattern would tend to bring more isotopically more negative winter precipitation to the region and could account for the 0.8‰ offset in δ18Oc data.

A lacustrine carbonate record of Holocene seasonality and climate

Annually laminated (varved) Holocene sediments from Derby Lake, Michigan, display variations in endogenic calcite abundance reflecting a long-term (millennial-scale) decrease in burial punctuated with frequent short-term (decadal-scale) oscillations due to carbonate dissolution. Since 6000 cal yr B.P., sediment carbonate abundance has followed a decreasing trend while organic-carbon abundance has increased. The correlation between organic-carbon abundance and the sum of March-April-October-November insolation has an r 2 value of 0.58. We interpret these trends to represent a precession-driven lengthening of the Holocene growing season that has reduced calcite burial by enhancing net annual organic-matter production and associated calcite dissolution. Correlations with regional paleoclimate records suggest that changes in temperature and moisture balance have impacted the distribution of short-term oscillations in carbonate and organic-matter abundance superimposed on the precession-driven trends.

Environmental change controls of lacustrine carbonate, Cayuga Lake, New York: Comment and Reply

Dated sediment cor es from Cayuga Lake, New York State, document that biologically mediated precipitation of calcite has been contr olled by environmental change, both natural and anthropogenic, overthe past 10000 yr. During the Holocene Hypsithermal (~9–4 ka [14C] ), Milankovitch for cing of summerinsolation in the Northern Hemisphere resulted in a broad increase (to 55%), then decrease (to <5%), of calcite content in bottom sediment. Warmer summers resulted in earlieronset of thermal stratification of the watercolumn, which increased the duration of primar y production as well as the abundance of picoplankton, which in turn increased the amount of calcite pr ecipitated. At the end of the Hypsithermal ca. 3500 yrago, global cooling greatly reduced the amount of calcite pr ecipitated. However , since A.D. 1940, calcite contents in Cayuga Lake sediments have risen up to ~20%. One hypothesis is that this recent increase in calcite is the r esult of cultural eutrophication (nutrient loading). However, this rise in calcite also closely tracks the anthr opogenic rise of atmospheric carbon dioxide, suggest ing a possible link to global envir onmental change. Fur ther research on hard-waterlake basins will be needed to test which of these two hypotheses is corr ect. Figure 1. Left: Index map of eastern Fing er Lakes region, New York State; stud y area is in southern half of Ca yuga Lake . Right: Bathymetr y of southern half of Ca yuga Lake illustrating ore locations; bathymetr y is fr om Mullins et al. (1996). environment around their cells, leading to epi cellular precipitation of calcite. Hodell et al. (1998) documented that calcite precipitation in nearby Lake Ontario is highly correlated to lake temperature, but is also dependent upon primary productivity and the abundance of picoplankton. METHODS Sediment cores were collected from two localities in Cayuga Lake: (1) near the area of maxi mum water depth in the southern half of the lake, and (2) at the southern terminus of the lake near Ithaca (Fig. 1). At the deep-water site (CL-1), a 5.4-m-long piston core and a 1 -m-long box core were collected from surface vessels. At the southern end of the lake (CLI -2), a 15-m-long sediment core was collected by hand using a 3-cm-diameter soil sampler . Age control for the two long cores was estab lished by radiocarbon dating of terrestrial or ganic material (with the exception of one bulk or ganic date; Table 1). Age models for the cores were developed by linear extrapolation between radio carbon data points, which were then converted to calendar years following Bartlein et al. (1995). Age control for the box core is based on identifi cation of a bomb spike in 137Cs profile data (A.D. 1963) at 17–18 cm below the lake floor , as well as 210Pb data. (Tom Kraemer , U.S. Geological Survey , 1996, written commun.). The 1-m-long box core from site CL -1 was sampled at 1 cm intervals (average ~2 yr), whereas the 5.4 -m-long piston core from this site was sampled every 10 cm (average ~135 yr). The 15-m-long core from site CLI -2 was sampled at a 25 cm interval, or approximately every 180 yr . Subsamples ( n = 214) were then analyzed for dry weight percent total or ganic matter and total car bonate content by loss on ignition at 550 °C and 1000°C, respectively (Dean, 1974); based on replicate analyses, error is <0.5%. Correlation coef ficients were calculated (Pearson’ s r) that produce a standardized statistic (+1 to –1) that is not scale dependent. Holocene records were digitized at 500 yr intervals over the past 10000 yr, whereas data sets covering the past 200 yr were digitized at an average of 4.4 yr . RESULTS Shallow-Water Site CLI-2 The 15-m-long sediment core recovered from the southern terminus of Cayuga Lake (Fig. 1) consists of an 8-m-thick sequence of light gray marl (>30% calcium carbonate) overlain by 6 m, and underlain by 1 m, of dark-gray mud (Fig. 2A). Oogonia (reproductive or gans of the aquatic macrophyte Chara) are present throughout the length of the core, indicating that this site has remained within the shallow photic zone for at least the past 1 1000 14C yr. Accumulation rates in the core vary from ~100 to 300 cm/1000 yr . When plotted against a radiocarbon time scale, results from core CLI-2 (Fig. 2A) indicate that marl deposition occurred from ca. 10.3 ka (end of he Younger Dryas cold interval) to ca. 3.5 ka (end of the Holocene Hypsithermal warm inter val). Maximum calcite content of 55% in the marl at ca. 7.2 ka ( 14C) drop to values of <5% in the overlying dark gray mud. The overall curve for calcite in core CLI -2 displays a broad in crease then decrease throughout the marl, al though smaller scale anomalies are also apparent (Fig. 2A). However , total organic matter (TOM) values are relatively invariant. Deep-Water Site CL-1 Sediments recovered in the 5.4 -m-long piston core at this deep-water (108 m) site consist of laminated, gray-brown to gray-black muds. Radiocarbon data indicate that the sediments in this core extend back to ca. 7.3 ka ( 14C) (Fig. 2B) at an essentially linear accumulation rate of ~80 cm/1000 yr . From ca. 7.3 to 3.4 ka ( 14C), calcite content varies from a maximum of 40% at 6.6 ka to a min imum of 8% at 3.4 ka (Fig. 2B). However , during the past 3400 yr ( 14C), calcite contents have always been less than a background level of 5%. This drop in calcite accumulation in core CL-1 at 3.4 ka (14C) (Fig. 2B) coincides with the cessation of marl deposition at site CLI-2 at ca. 3.5 ka (14C) (Fig. 2A). TOM values in core CL -1 display an up-core increase from ~6% to 10%. Results from the box core recovered at site CL-1 indicate that between 100 and 70 cm below the lake floor , calcite values are always <5% (Fig. 3), as they have been since ca. 3.4 ka ( 14C). However , between 70 and 28 cm, calcite contents increase to 10%, and in the upper 28 cm of box core CL-1, calcite contents rise rapidly from 8% to 20% (Fig. 3). TOM values similarly show an