An alternative suggestion for the Pliocene onset of major northern hemisphere glaciation based on the geochemical provenance of North Atlantic Ocean ice-rafted debris
An alternative suggestion for the Pliocene onset of major northern hemisphere glaciation based on the geochemical provenance of North Atlantic Ocean ice-rafted debris
复制标题
DOI：
10.1016/j.quascirev.2013.06.004
复制
发表时间：
2013-09-01
影响因子：
4
通讯作者：
Raymo, Maureen E.
中科院分区：
文献类型：
--
作者：
Bailey, Ian;Hole, Georgia M.;Raymo, Maureen E.
272万年前北欧海和北大西洋次极地丰富的冰筏碎屑(IRD)沉积被认为记录了北半球主要冰川(NHG)的上新世开始，这是由于北美Laurentide、斯堪的纳维亚和格陵兰冰盖在海洋同位素阶段G6期间同步推进到其海洋崩解边缘。来自北欧海域的大量海洋和陆地记录表明，格陵兰岛和斯堪的纳维亚半岛上广阔的冰盖增加了IRD对这些海域的输入，从2.72 Ma。然而，IRD在亚极地北大西洋的沉积追踪到的北美冰盖扩张的时间不太清楚，因为欧洲和北美都是该地区冰山的潜在来源。此外，对北美陆地冰盖的宇宙成因测年表明，Laurentide冰盖直到2.4 Ma+/-0.14 Ma才延伸到类似北纬39度的位置，这至少是在2.72 Ma主要IRD沉积开始后的180ka。为了解决这个问题，我们首次详细分析了在亚极地北大西洋MISG6和100(类似于2.72-2.52 Ma)之间沉积的单个砂粒大小的IRD的地球化学来源。采用激光烧蚀铅(铅)同位素分析方法对冰排(>；150亩)长石单颗粒进行了物源分析。为了追踪在上新世NHG增强(In Hg)期间，北大西洋首次出现与主要NHG一致的冰流环境，我们调查了在DSDP 611站位沉积的长石的铅同位素组成(Pb206/Pb204，Pb207/Pb204，Pb208/Pb204)何时首次与MIS100(2.52 Ma)期间IRD的测定结果相似，MIS100(2.52 Ma)是最古老的冰川，有令人信服的证据表明北美普遍存在冰川作用。在距今2.72 Ma的冰期期间，611站位存在第四纪IRD通量，但我们发现这种IRD的来源并不是恒定的。相反，我们发现研究地点的IRD的铅同位素组成与主要的NHG不一致，直到MISG2(2.64 Ma)。我们假设，在MISG2之前，北大西洋的IRD沉积主要是来自格陵兰岛和斯堪的纳维亚半岛的冰山崩解。我们进一步认为，在2.64 Ma之前，美国东北部的大陆冰的触地线可能没有延伸到大陆架，并在冰川时期将大量冰山崩解到北大西洋。(C)2013爱思唯尔有限公司。保留所有权利。
The onset of abundant ice-rafted debris (IRD) deposition in the Nordic Seas and subpolar North Atlantic Ocean 2.72 millions of years ago (Ma) is thought to record the Pliocene onset of major northern hemisphere glaciation (NHG) due to a synchronous advance of North American Laurentide, Scandinavian and Greenland ice-sheets to their marine calving margins during marine isotope stage (MIS) G6. Numerous marine and terrestrial records from the Nordic Seas region indicate that extensive ice sheets on Greenland and Scandinavia increased IRD inputs to these seas from 2.72 Ma. The timing of ice-sheet expansion on North America as tracked by IRD deposition in the subpolar North Atlantic Ocean, however, is less clear because both Europe and North America are potential sources for icebergs in this region. Moreover, cosmogenic-dating of terrestrial tills on North America indicate that the Laurentide Ice Sheet did not extend to similar to 39 degrees N until 2.4 +/- 0.14 Ma, at least 180 ka after the onset of major IRD deposition at 2.72 Ma. To address this problem, we present the first detailed analysis of the geochemical provenance of individual sand-sized IRD deposited in the subpolar North Atlantic Ocean between MIS G6 and 100 (similar to 2.72-2.52 Ma). IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single ice-rafted (>150 mu m) feldspar grains. To track when an ice-rafting setting consistent with major NHG first occurred in the North Atlantic Ocean during the Pliocene intensification of NHG (iNHG), we investigate when the Pb-isotope composition (Pb-206/Pb-204, Pb-207/Pb-204, Pb-208/Pb-204) of feldspars deposited at DSDP Site 611 first resembles that determined for IRD deposited at this site during MIS 100 (2.52 Ma), the oldest glacial for which there exists convincing evidence for widespread glaciation of North America. Whilst Quaternary-magnitude IRD fluxes exist at Site 611 during glacials from 2.72 Ma, we find that the provenance of this IRD is not constant. Instead, we find that the Pb-isotope composition of IRD at our study site is not consistent with major NHG until MIS G2 (2.64 Ma). We hypothesise that IRD deposition in the North Atlantic Ocean prior to MIS G2 was dominated by iceberg calving from Greenland and Scandinavia. We further suggest that the grounding line of continental ice on Northeast America may not have extended onto the continental shelf and calved significant numbers of icebergs to the North Atlantic Ocean during glacials until 2.64 Ma. (c) 2013 Elsevier Ltd. All rights reserved.