Anthropogenic climate change has driven Lake Superior productivity beyond the range of holocene variability: an organic and stable isotopic study of Human impacts on a pristine biogeochemical system

Abstract

Recent studies have noted that changes in Lake Superior's physical, chemical and biological processes are apparent - including a warming of the surface waters at a rate twice as great as the surrounding airshed in the last 20 years. These changes are often difficult to perceive as cause for concern when not placed within a historical context. In this study, bulk C and N elemental abundance and stable isotope analysis of sediments from three piston and corresponding gravity cores, representing a record of lake-wide paleoproductivity trends spanning the Holocene, allows for the historical comparison with recent (1800 A.D. to present) productivity trends. Overall, Lake Superior experiences a slow, steady increase in productivity consistent with the concept of `natural' eutrophication, which is characterized by gradual increases in TOC and TON, as well as the steady <super>13</super>C-enrichment of bulk sedimentary organic carbon and <super>15</super>N-enrichment of bulk sedimentary organic nitrogen compositions. Over the last 200 years bulk concentrations and stable isotope compositions of carbon and nitrogen from eight multicores sampled at high resolution indicate that the Lake Superior basin has undergone productivity changes in the last two centuries (1800 to present) which are unique in the context of the Holocene. Overall, lake-wide sedimentary bulk organic proxy data show increasing primary production between 1900 and present, as indicated by an ~2 &#8240 increase in &#948;<super>13</super>C_org. The most recent increases in productivity are likely a response to increasing water temperatures and longer stratified periods reported in Lake Superior. Down-core variations in the &#948;<super>13</super>C composition of algal-derived short-chain n-alkanes do not exhibit the same trend as that observed for bulk sedimentary organic matter (&#948;<super>13</super>C_org). The &#948;<super>13</super>C of bulk sedimentary organic matter shows systematic <super>13</super>C-enrichment over the last ~9000 years, while the &#948;<super>13</super>C values of aquatic derived n-alkanes exhibit a systematic <super>13</super>C-depletion to present-day. Down-core variation in &#948;<super>13</super>C values of n-alkanes likely reflect multiple isotope effects associated with carbon partitioning and fractionation associated with the biosynthesis of n-alkanes.