NOAA/WDS Paleoclimatology - Gulf of Alaska Alkenone paleotemperatures, d18Oseawater reconstructions, benthic oxygen isotopes and benthic faunal data for the past 18,000 years

Marine sediments from the North Pacific document two episodes of expansion and strengthening of the subsurface oxygen minimum zone (OMZ) accompanied by seafloor hypoxia during the last deglacial transition. The mechanisms driving this hypoxia remain under debate. We present a new high-resolution alkenone palaeotemperature reconstruction from the Gulf of Alaska that reveals two abrupt warming events of 4–5 degrees Celsius at the onset of the Bølling and Holocene intervals that coincide with sudden shifts to hypoxia at intermediate depths. The presence of diatomaceous laminations and hypoxia-tolerant benthic foraminiferal species, peaks in redox-sensitive trace metals, and enhanced 15N/14N ratio of organic matter, collectively suggest association with high export production. A decrease in 18O/16O values of benthic foraminifera accompanying the most severe deoxygenation event indicates subsurface warming of up to about 2 degrees Celsius. We infer that abrupt warming triggered expansion of the North Pacific OMZ through reduced oxygen solubility and increased marine productivity via physiological effects; following initiation of hypoxia, remobilization of iron from hypoxic sediments could have provided a positive feedback on ocean deoxygenation through increased nutrient utilization and carbon export. Such a biogeochemical amplification process implies high sensitivity of OMZ expansion to warming.

北太平洋海底沉积物记录了末次冰消过渡期内，地下氧最小层（OMZ）两次扩张增强的事件，且伴随海底缺氧现象。该缺氧现象的驱动机制目前仍存在争议。本研究基于阿拉斯加湾的新数据，构建了高分辨率烯酮古温度重建序列，结果显示在博林期与全新世初期，发生了两次4~5摄氏度的突发性升温事件，这两次升温与中层水深的突发性缺氧转变相吻合。硅藻纹层、耐缺氧底栖有孔虫物种的存在、氧化还原敏感痕量金属的峰值，以及有机质15N/14N比值的升高，共同表明该现象与高输出生产力存在关联。伴随最严重脱氧事件的底栖有孔虫18O/16O比值降低，表明地下水温升高可达约2摄氏度。我们推断，突发性升温通过降低氧气溶解度、以及通过生理效应提升海洋生产力，触发了北太平洋氧最小层的扩张；在缺氧现象启动后，缺氧沉积物中铁的再活化可通过提升营养盐利用效率与碳输出，对海洋脱氧过程产生正反馈效应。这种生物地球化学放大过程表明，氧最小层的扩张对升温具有极高的敏感性。