Environmental change controls of lacustrine carbonate, Cayuga Lake, New York

Dated sediment cores from Cayuga Lake, New York State, document that biologically mediated precipitation of calcite has been controlled by environmental change, both natural and anthropogenic, over the past 10 000 yr. During the Holocene Hypsithermal (~9–4 ka [14C]), Milankovitch forcing of summer insolation 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 earlier onset of thermal stratification of the water column, which increased the duration of primary production as well as the abundance of picoplankton, which in turn increased the amount of calcite precipitated. At the end of the Hypsithermal ca. 3500 yr ago, global cooling greatly reduced the amount of calcite precipitated. 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 result of cultural eutrophication (nutrient loading). However, this rise in calcite also closely tracks the anthropogenic rise of atmospheric carbon dioxide, suggesting a possible link to global environmental change. Further research on hard-water lake basins will be needed to test which of these two hypotheses is correct. Figure 1. Left: Index map of eastern Finger Lakes region, New York State; study area is in southern half of Cayuga Lake. Right: Bathymetry of southern half of Cayuga Lake illustrating core locations; bathymetry is from Mullins et al. (1996). environment around their cells, leading to epicellular 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 maximum 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 established by radiocarbon dating of terrestrial organic material (with the exception of one bulk organic date; Table 1). Age models for the cores were developed by linear extrapolation between radiocarbon data points, which were then converted to calendar years following Bartlein et al. (1995). Age control for the box core is based on identification 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 organic matter and total carbonate content by loss on ignition at 550 °C and 1000 °C, respectively (Dean, 1974); based on replicate analyses, error is <0.5%. Correlation coefficients 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 10 000 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 organs 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 11000 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 the Younger Dryas cold interval) to ca. 3.5 ka (end of the Holocene Hypsithermal warm interval). 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 increase then decrease throughout the marl, although 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 minimum 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