Sediment Trap Time Series of GDGT and alkenone flux in the Gulf of Mexico

The tetraether index of C86 (TEX86) and alkenone unsaturation index (Uk37Õ) molecular biomarker proxies have been broadly applied in down-core marine sediments to reconstruct past sea surface temperature (SST). Although both TEX86 and Uk37 have been interpreted as proxies for mean annual SST throughout the global ocean, regional studies of glycerol dibiphytanyl glycerol tetraethers (GDGT)s and alkenones in sinking particulate matter (SPM) are required to understand the influence of seasonality, depth distribution and diagenesis on downcore variability. USGS scientists measured GDGT and alkenone flux, as well as the TEX86 and Uk37Õ indices in a 4-year sediment trap time series (2010-2014) in the northern Gulf of Mexico (nGoM), with weekly-to-monthly resolution, and compared these data with core-top sediments at the same location. GDGT and alkenone fluxes do not show a consistent seasonal cycle; however, the largest flux peaks for both occurs in winter. Uk37 co-varies with SST over the 4-year sampling interval, but the U-SST relationship in this dataset implies a smaller slope or non-linearity at high temperatures when compared with existing calibrations. Furthermore, the flux-weighted Uk37 value from sinking particles is significantly lower than that of underlying core-top sediments, suggesting preferential diagenetic loss of the tri-unsaturated alkenone in sediments. TEX86 does not co-vary with SST, suggesting production in the subsurface ocean. The flux-weighted mean TEX86 matches that of core-top sediments, suggesting that sedimentary TEX86 in the Gulf of Mexico reflects local autochthonous production. We explore these potential sources of uncertainty in both proxies in the GoM, but demonstrate that they show nearly identical trends in 20th century SST, despite these factors.

C86四醚指数（TEX86）与烯酮不饱和度指数（Uk'37）分子生物标志物代用指标，已被广泛应用于海底岩心沉积物，以重建过去的海表温度（SST）。尽管全球海洋中的TEX86与Uk'37均被解读为年平均海表温度的代用指标，但仍需对沉降颗粒物（SPM）中的甘油二烷基甘油四醚（GDGT）与烯酮开展区域研究，方能厘清季节变化、深度分布及成岩作用对岩心记录变异性的影响。美国地质调查局（USGS）科研人员于墨西哥湾北部（nGoM）开展了一项为期四年（2010-2014年）的沉积物捕集器时间序列研究，以周-月分辨率测定了GDGT与烯酮通量，以及TEX86和Uk'37指数，并将所得数据与同一站位的岩心表层沉积物进行了对比。GDGT与烯酮通量未呈现统一的季节周期，但二者的最大通量峰值均出现在冬季。Uk'37在四年采样周期内与海表温度呈共变关系，但本数据集所得到的Uk'37与海表温度的响应关系表明，相较于现有校准方案，高温区间内的斜率更小或呈现非线性特征。此外，沉降颗粒物的通量加权Uk'37均值显著低于下伏岩心表层沉积物，这暗示沉积物中三不饱和烯酮发生了选择性成岩降解。TEX86并未与海表温度呈现共变关系，表明其产生于次表层海洋。沉降颗粒物的通量加权平均TEX86数值与岩心表层沉积物的结果相匹配，这说明墨西哥湾海域的沉积TEX86能够反映当地的原地生源产出。本研究探讨了墨西哥湾海域两类代用指标的上述潜在不确定性来源，但研究表明，尽管存在上述影响因素，二者在20世纪海表温度重建中均呈现近乎一致的变化趋势。