Distribution and composition of organic matter in surface sediments of coastal Southeast Alaska

Total organic carbon (TOC) and biogenic silica (opal) content, elemental (C/N) and isotopic (delta C-13, delta N-15) composition of organic matter and the content of lipid biomarkers derived from both marine and terrestrial sources constrain relative contributions from marine productivity and continental erosion to surface sediments throughout coastal SE Alaska (54 degrees N to 61 degrees N). TOC and opal content are very high (up to 8% and 33% by weight, respectively) in fjords and inlets south of Icy Strait (similar to 58 degrees N) and uniformly low at offshore sites to the south, and at both offshore and inland sites to the north (averaging 0.6 +/- 0.3% and 2.3 +/- 1.8%, respectively). TOC and opal mass accumulation rates (MARS, based on bulk density and Pb-210-derived sediment MAR) suggest dilution with terrigenous, inorganic detrital materials accounts for the low concentrations of both biogenic phases in sediments from the glacial tidewater fjords of Muir and Yakutat Bays but not elsewhere. C/N, delta C-13, and delta N-15 indicate a dominant marine origin for organic matter deposited at most sites. This conclusion implicates elevated primary productivity in inland waters to the south with diatoms, based on opal results, being the dominant contributor. A very significant terrestrial organic fraction (25-50%) is contained in sediments deposited on the continental shelf to the north of 58 degrees N. Hydrocarbon biomarkers indicate the terrestrial fraction in sediments from this region is represented by old organic matter (kerogen) likely contained within riverborne particles eroding from now heavily glaciated adjacent landscapes. In sediment to the south, the terrestrial fraction is traced to modern soil organic matter eroded from the now non-glaciated, heavily forested adjacent landscape. Our study provides a framework to guide future investigations of short(anthropogenic) to long- (Holocene) term environmental and/or climate change in this region through down-core, stratigraphic analysis. (c) 2009 Elsevier Ltd. All rights reserved.