A pre-bomb radiocarbon freshwater reservoir correction for the Laurentian Great Lakes

Abstract

Reservoir 14C age offsets track past changes in atmospheric carbon content, air–water gas exchange, and freshwater circulation in water bodies. Determination of a freshwater reservoir correction (FWRC) is a crucial step in age-model development to provide accurate climate-proxy records. We employ published, recent water column 14C measurements from Lake Superior, and reassess unpublished and published 14C measurements of live-collected, pre-bomb bivalve shells dated from lakes Michigan, Huron, Erie, and Ontario, to provide the most comprehensive, consistent snapshot of the mid twentieth century (pre-bomb) FWRC for the Laurentian Great Lakes assembled to date. We also use mollusk/ostracod and wood shell ages from the bottom 60 cm of a 1218 cm core from Lake Erie’s eastern basin to estimate a late Holocene FWRC. The FWRC for all live-collected samples were recalculated in a consistent-manner using the IntCal20 atmospheric 14C curve. The revised corrections provide modest revisions (± 10%) of published values. The revised FWRC values document a generally increasing trend along the circulation pathway from Lake Superior to Michigan, Huron, and Erie with average FWRC in 14C years of 206 ± 55 for Lake Superior, 247 ± 81 for Lake Michigan, 365 ± 41 in Green Bay, 340 ± 72 for northern Lake Huron, 321 ± 71 in Georgian Bay, to 443 ± 62 in southern Lake Huron. For Lake Erie, the average FWRC for the western basin is 455 ± 52 14C years, for the central basin 676 ± 52 14C years, and 748 ± 76 14C years for the eastern basin. In Lake Ontario, the average FWRC for the western part of the lake is 540 ± 55 14C years, smaller than the value observed for Lake Erie. The late Holocene FWRC for eastern Lake Erie is estimated between 440 ± 124 14C years and 306 ± 124 14C years, which is smaller than the pre-bomb value. This study provides a conceptual framework and statistical model to explain the observed pattern of FWRC in the Great Lakes. Higher FWRC values are encountered in basins surrounded by more carbonate bedrock and higher inflow rates, mostly from upstream lakes. On a basin-wide scale, there is a statistically significant positive linear relation for the observed FWRC, which depends on the fractional area of carbonate bedrock within each Great Lake watershed and the inflow rate to each basin, accounting for essentially all the variance in the data.