Enhanced Arctic Stratification during Mid Pliocene Warm Period

This study proposes that the water column stratification strengthens at the Yermak Plateau during warmer periods of MPWP and weakened during colder periods based on the relative nutrient utilization at the surface. The relative nutrient utilization is reconstructed using nitrogen isotope (δ15N) of sedimentary organic matter (SOM), where we have also checked for the provenance and the burial diagenesis. The sediment core is collected during ODP expedition 151 from Hole 910C, Yermak Plateau, Arctic Ocean. The water column depth at the study site is 556.4 m. We have measured the δ15N values of SOM to reconstruct the relative nutrient utilization. The CaCO3 %, Mass accumulation rate of CaCO3 (MAR), and Ba/Al are used to reconstruct the surface water productivity. The carbon isotope (δ13C) and the elemental concentration (total organic carbon (TOC) and total nitrogen (TN)) of SOM are measured to determine the provenance and diagenetic alteration at the study site. The provenance of the organic matter is determined from δ13C versus C/N ratio. While, the diagenetic alteration is observed from the cross plot between δ15N vs TOC, TN, and C/N. From this data set, we found that the origin of the organic matter is mostly from the marine environment with a negligible diagenetic alteration. The high δ15N values with high productivity indicate the maximum consumption of nutrients during the interglacial periods, which suggests that the limited nutrient supply to the surface could be due to the strong stratification. Whereas during the colder period the low δ15N values with low productivity indicate low relative nutrient utilization and it implies the repletion of nutrient at the surface with new nitrate could be due to the weak stratification. This study finds that the strong stratification during the warmer period at the Yermak Plateau could be due to the freshening of the surface layer via more sea ice melt, river runoff and, net precipitation. The weak stratification during the colder period could be due to the lowering of salinity gradient due to brine rejection during sea ice formation.本研究基于表层相对养分利用效率的变化，提出北极耶尔马克高原的水体层化（water column stratification）在中更新世暖期（MPWP）增强，而在冷期减弱。研究利用沉积有机质（sedimentary organic matter, SOM）的氮同位素（δ¹⁵N，nitrogen isotope）重建相对养分利用效率，并同时验证了有机质的物源与埋藏成岩作用。沉积岩芯采自北冰洋耶尔马克高原910C站位，该岩芯取自大洋钻探计划（Ocean Drilling Program, ODP）第151航次，研究站位的水体深度为556.4米。本研究测定了沉积有机质的δ¹⁵N值以重建相对养分利用效率；采用碳酸钙百分含量（CaCO₃ %）、碳酸钙质量堆积速率（mass accumulation rate of CaCO₃, MAR）以及钡铝比（Ba/Al）重建表层水生产力；同时测定了沉积有机质的碳同位素（δ¹³C，carbon isotope）与元素浓度，包括总有机碳（total organic carbon, TOC）和总氮（total nitrogen, TN），以判定研究区的物源与成岩蚀变情况。其中，有机质物源通过δ¹³C与C/N比值的交会图（cross plot）进行判定，而成岩蚀变则通过δ¹⁵N分别与TOC、TN及C/N的交会图识别。通过本数据集的分析可知，研究区的有机质主要源自海洋环境，且成岩蚀变程度可忽略不计。间冰期时高δ¹⁵N值对应高生产力，表明此时养分消耗达到峰值，暗示表层养分供给受限是由强烈的水体层化所致；而冷期时低δ¹⁵N值对应低生产力，反映相对养分利用效率较低，表明表层可通过新补充的硝酸盐维持养分充足，这一现象可归因于较弱的水体层化。本研究进一步发现，耶尔马克高原暖期的强水体层化可能源于海冰融化、河流径流增加与净降水增多共同导致的表层水体淡化；冷期的弱水体层化则可能归因于海冰形成过程中排盐作用引发的盐度梯度降低。