Amino acids and hexosamines in the Hess Rise core during the past 220,000 years

Core sediment samples collected from the Hess Rise, North Pacific, were analyzed for 20 common amino acids (AA) and two hexosamines (HA) to understand the relation between glacial–interglacial variations and deposition/preservation of sedimentary organic matter (OM). The sediments are predominantly carbonaceous (carbonates 35–80%). AA-based parameters—aspartic acid/glycine ratio and serine+threonine relative mole content—suggest that calcareous plankton was the major source of OM in these sediments. This inference is supported by the similarity in distribution patterns of AA and HA contents with that of organic carbon. Low values of AA/HA and glucosamine/galactosamine ratios (average 4.4 and 1.1, respectively) imply that much of the planktonic OM was replaced by microbial OM. The relative molar concentration of two nonprotein AA (β-alanine and γ-aminobutyric acid) varied with age of sediments; i.e., they were less abundant in recent sediments and more abundant in the oldest sediments. This trend is an indicator of extremely slow but continuous enzymatic degradation of proteinaceous OM within the sediments. So far, bulk OM has been believed to be one of the best proxies for estimation of primary productivity. However, it may be an underestimate, even for the late Quaternary sediments. Comparison of AA and HA content variations with SPECMAP stack revealed their enhanced deposition and preservation during glacial periods relative to interglacial periods. This, in turn, affected not only the planktonic production in surface waters but also the benthic community, including bacteria on the seafloor.