以有機固醇類作為古氣候代用指標重建中更新世變遷時期鄂霍次克海之氣候歷史

Abstract

生物生產力受氣候與環境因子控制，並在氣候系統回饋機制和全球碳循環中扮演重要角色。本研究利用鄂霍次克海中部的海洋岩芯材料MD01-2414 (53◦11.77′ N, 149◦34.80′ E)，透過氣相層析質譜儀分析沉積物中六種常見的有機固醇類濃度 (β-sitosterol, stigmasterol, campesterol, dinosterol, brassicasterol and 24-methylene cholesterol)，作為代用指標重建120萬至60萬年來鄂霍次克海以及鄰近東北部西伯利亞地區陸地生產力。 結果顯示陸相與海相來源固醇類皆有明顯的冰期/間冰期週期變化，在間冰期濃度較高。本文首先探討影響沉積物中固醇類濃度的因素，藉由陸相/海相來源比例變化以及固醇類濃度隨深度遞增趨勢，顯示在本研究地點，生物生產力變化的影響大於有機質保存以及陸地沉積物傳輸量。而後本文利用固醇類通量作為生產力指標討論氣候事件：根據固醇類以及前人研究之鄰近地區海表溫與海冰紀錄，西北太平洋地區經歷較為寒冷的間冰期MIS 23且此寒冷狀態延續至MIS 22。而在MIS 31-32時期，固醇類以及前人研究之鄰近地區海表溫、東北西伯利亞陸地湖泊紀錄，顯示此區域於MIS 32初期開始升溫，並於MIS 32末期達最溫暖狀態，此變化早於前人研究之超級間冰期MIS 31紀錄，也領先過往推測可能為控制因素的南、北半球日照量峰值。 本文探討沉積物中固醇類化合物作為生物生產力代用指標之可行性，並藉以重建西北太平洋和東北西伯利亞環境，顯示此區域陸地與海洋生產力於間冰期時增加，並在900 ka左右經歷寒冷事件以及在MIS 32經歷較為溫暖的冰期。

Biological productivity, controlled by climate and environmental factors, plays an important role in climate feedback mechanisms and the global carbon cycle. In order to provide more data for discussion of the interactions, in this study, six sterol compounds (β-sitosterol, stigmasterol, campesterol, dinosterol, brassicasterol and 24-methylene cholesterol) from the core MD01-2414 (53◦11.77′ N, 149◦34.80′ E) from the central Okhotsk Sea were measured for 1200-60 ka by using gas chromatography-mass spectrometer. These sterols derived from different environments were used as paleoproductivity proxies to reconstruct productivity in the central Okhotsk Sea and the northeast Siberia. Our results show that all the sterols generally follow the global glacial/interglacial cycles during the 1200-60 ka, with higher values in interglacial and lower ones in glacial periods. Ba/Ti and C/N ratio indicating marine versus terrestrial sources change and the increasing trend in all the sterol concentrations show that productivity is the major control on sedimentary sterols rather than transportation and preservation in this study site, confirming the representativeness of sterols as proxies of biological productivity. According to sterol fluxes in this study, sea surface temperature records from the northwest Pacific and sea ice proxy from the Bering Sea, a “failed” interglacial, MIS 23, was observed and the cold condition remained in MIS 22. The sterol data compiled with sea surface temperature records of the Okhotsk Sea and northwest Pacific, and pollen, temperature and sedimentary faces of sediments from Lake El’gygytgyn located in the Far East Arctic region show a warming event at the onset of MIS 32 and reach the warmest at late MIS 32, which is prior to other records of the “super-interglacial”, MIS 31, and to the maximum value of boreal summer insolation and austral summer insolation. In sum, transportation of terrestrial detritus and preservation of organic matter were considered in order to confirm the feasibility of sedimentary sterol as a productivity proxy. The results of sterols indicate higher marine and land productivity in interglacials in northwest Pacific and northeast Siberia. A cooling events around 900 ka and a warming trend in the glacial period, MIS 32, were found in the sterol data and also in records from previous studies.