Most previous studies on the effects of acid rain have been conducted in areas with low acid-neutralizing, or buffering, capacity. In contrast, significantly less study has occurred in calcareous terrane that has high-buffering-capacity and that has traditionally been thought to be “insensitive” to the effects of acid rain. To address this fundamental inequity we have conducted a comprehensive study of near-surface (<1 m), historic age (<200 yr) sediments in the Finger Lakes of central New York State, a high buffering capacity area that has received significant quantities of acid rain. In particular, we have focused on authigenic calcite contained in historic bottom sediment as well as water-column geochemistry between 1972 and 1999.
Results indicate that the open-water precipitation of calcite (“whitings”) returned to 7 of the 11 Finger Lakes during historic time following an ∼4500 14C yr hiatus. Only those lakes with calcite in the near-surface sediment have calcareous soils and/or limestone outcrops in their drainage basins and summer surface waters that are supersaturated with respect to calcite. Water-column data indicate that the saturation state of summer surface waters has increased in at least seven Finger Lakes between 1972 and 1999 and that calcium loading to lakes lacking calcite has also increased over the same time interval.
We hypothesize that acid rain has increased carbonate weathering in the Finger Lakes region, leading to an increase in supersaturation of summer surface waters with respect to calcite. The precipitation of calcite in summer is triggered by evaporation, temperature increase, and/or photosynthetic removal of carbon dioxide. This new “chemical weathering” hypothesis contrasts with, but is not mutually exclusive of, the traditional “cultural-eutrophication” model that suggests that increased nutrient loading, biological productivity, and CO2 removal are the primary causal mechanisms for historic calcite precipitation. Stable carbon isotope and water-column geochemical data from the Finger Lakes indicate decreasing levels of primary production since at least the 1960s, and there are statistically significant correlations between CO2, SO2, and NOx emissions and calcite content of bottom sediments.
Historic calcite precipitation in the Finger Lakes began as early as the 1820s, suggesting that naturally acidic rain falling on freshly deforested and tilled landscapes increased chemical weathering and brought these lakes to supersaturation. There was also a rapid increase in calcite precipitation beginning in ca. 1940 (coincident with increased industrialization and acidic rainfall associated with World War II) that continued into the 1990s. During the mid- to late 1990s, calcite precipitation decreased in the Finger Lakes following the introduction of zebra mussels. Unlike regions with low acid-neutralizing-capacity where acid rain results in the “acidification” of lakes, acidic deposition across well-buffered terrane may result in the “alkalization” of lakes.