Stable carbon and hydrogen isotopic compositions of palmitic acid in surface sediment from Baffin Bay and the Labrador Sea

Palmitic acid (PA) is ubiquitous in the biosphere and its hydrogen isotopic composition (δ2HPA) was proposed as a potential paleoenvironmental proxy for salinity, with δ2HPA values increasing with salinity. In this study, we analyzed 40 surface sediment samples from Baffin Bay and the Labrador Sea to examine the isotopic composition of PA in relation to local environmental variables, including salinity. In contrast to expectations, our results show a negative relationship between the δ2HPA and sea-surface salinity, raising questions about its pertinence/usefulness as a salinity proxy. Instead, our results suggest that the relative abundance of distinct organisms that employ different metabolisms is key in determining the hydrogen isotopic fractionations in PA. Whereas we show that PA is mostly produced through photoautotrophic metabolisms by diatoms and dinoflagellates, varying contributions from heterotrophic metabolisms may obscure the stable isotope composition of PA. Surprisingly, we found no correlation between the stable carbon isotopic composition of the sedimentary organic matter (δ13Corg) and palmitic acid (δ13CPA), implying major differences in either the dominant organisms producing sedimentary PA or in carbon isotope fractionation during lipid biosynthesis. We also found that the presence of extended sea-ice cover leads to enriched carbon and hydrogen isotopic compositions in PA. These enriched values suggest heterotrophic biodegradation in the water column and/or in the sediment as well as an increase in grazing activities. We propose that sea-ice cover and surface water oxygenation modulate the relative impact of phototrophic and heterotrophic metabolisms, and therefore the isotopic composition of marine sedimentary PA.

棕榈酸（Palmitic acid, PA）广泛分布于生物圈中，其氢同位素组成（δ²HPA）曾被提议作为盐度相关的潜在古环境代用指标（paleoenvironmental proxy），且δ²HPA值随盐度升高而增大。本研究对采自巴芬湾与拉布拉多海的40个表层沉积物样品进行分析，以探究PA的氢同位素组成与包括盐度在内的当地环境变量之间的关联。与此前的预期相悖，本研究结果显示δ²HPA与海表盐度呈负相关关系，这对其作为盐度代用指标的适用性与应用价值提出了质疑。
与之相反，本研究结果表明，采用不同代谢模式的不同类群生物的相对丰度，才是决定PA中氢同位素分馏的关键因素。尽管本研究证实PA主要由硅藻（diatoms）与甲藻（dinoflagellates）通过光合自养代谢（photoautotrophic metabolisms）途径合成，但异养代谢（heterotrophic metabolisms）的不同贡献占比，可能会掩盖PA的稳定同位素组成特征。
令人意外的是，本研究未发现沉积有机质（sedimentary organic matter, δ¹³C_org）的稳定碳同位素组成与棕榈酸的碳同位素组成（δ¹³C_PA）之间存在相关性，这表明，要么生成沉积PA的优势生物类群存在显著差异，要么脂质生物合成（lipid biosynthesis）过程中的碳同位素分馏机制存在不同。此外，本研究还发现，长期海冰覆盖（extended sea-ice cover）的存在会使PA中的碳、氢同位素组成发生富集。这种同位素富集现象表明，水柱（water column）及/或沉积物中存在异养生物降解作用，同时生物摄食活动也有所增强。
本研究提出，海冰覆盖与表层水体充氧（surface water oxygenation）过程会调控光合自养与异养代谢的相对影响强度，进而改变海洋沉积PA的同位素组成。