Deciphering the Paleochemistry and Holocene Environmental Variability in Central New York: Different Perspectives from the Stable Carbon Isotopes of Organic Matter and Carbonates.

Abstract:

This study investigates environmental variability recorded in lacustrine and wetland sediments of Seneca Lake. Sediments display a range of stable isotopic and elemental chemistries suggesting there are changes in lake level, climate, or both during the Holocene in central New York. A ~13.5 m sediment core collected from the Catharine Creek wetland located south of Watkins Glen, NY at the southern tip of Seneca Lake was analyzed for total nitrogen (% TN), total organic carbon (% TOC), C/N ratios, δ¹³C of bulk organic matter (δ¹³C_org) and δ¹³C_cc and δ¹⁸O_cc of carbonate. There is little change in TOC with the exception of an organic-rich interval (~20% TOC) between 5.5 and 4.3 m. Between 13.5 and 6 m, the C/N ratios decrease gradually upcore, from ~40 to ~10 – 20 and is paralleled by an increase in δ¹³C_org values from ~ -27‰ to ~ -24‰. Between 6-5.5 m, δ¹³C becomes significantly more negative (~ -30‰). The δ¹³C_org increases (to -26‰) upcore from 5.5 to 2.4 m. High C/N values (ranging from 60 to 20) from 15.5 to 11.8 m are consistent with input of land plants. The C/N values from 11.8 to 2.6 m range from 10 – 20 and represents a mixed signal of both algal and land plant derived organic matter. Throughout the Holocene, there has been a steady shift from negative δ¹³C_org values to more positive values of organic matter, which is consistent with an increase in the abundance of C4 plants in the watershed. Shifts in C/N over the length of the core suggest changes in lake level. In contrast, carbonate isotopic values (δ¹³C_cc and δ¹⁸O_cc) do not covary and trends are consistent with a fresh-water, over-filled lake. In most lacustrine carbonate studies, calculated paleo-water temperatures from δ¹⁸O_cc are commonly used prima facie to reflect environmental variability and constrain temperatures. In this study, wide variations in the calculated paleo-water temperatures reflect multiple carbonate sources / fractionations invalidating their use as a first order indication of temperature. We interpret the vertical stratigraphic variations of both δ¹³C_org and δ¹³C_cc to reflect either multiple sources or changes in internal carbon cycling / processes or both driven by a complex history of lake level changes associated with isostatic rebound.