Assessing changes in temperature, ice volume and ice sheet dynamics across the Middle Pleistocene Transition

在过去的250万年里，地球气候的特点是北半球大型大陆冰盖的盛衰。在大约100万年前之前，冰期和间冰期的旋回以地球自转轴的倾斜（即倾角）的节奏行进，其变化周期为4.1万年。大约100万年前，冰期-间冰期旋回的长度变长，平均约为10万年。这种变化被称为中更新世转变（MPT），是古气候学中一个重要的未解决问题。为什么更新世中期冰期-间冰期旋回的频率突然从41- 100年转变为100年？冰盖的大小在MPT上变得更大，从而使北美冰盖的单个圆顶结合在一起形成了一个巨大的冰盖。这种冰的巨大体积和厚度使其对北方夏季日照的变化不太敏感，从而延长了冰期周期，因为冰盖在日照最大值和消冰期开始跳过岁差（~21 kyr）和/或倾角（~41 kyr）周期。阻碍我们进一步了解MPT的一个障碍是缺乏对发生在MPT上的冰体积变化的定量估计。造成这一缺陷的主要原因是底栖动物的氧同位素记录同时反映了温度和冰体积的变化。利用底栖生物的Mg/Ca，我们旨在反卷积底栖生物d18O信号，并提供与全球冰量相关的海水d18O变化的定量估计。随着北美冰盖在MPT之后体积和厚度的增加，冰盖的动力行为可能发生了变化。在末次冰期，Laurentide冰盖的动力学被称为海因里希层（Heinrich layers），它由沉积在北大西洋从拉布拉多延伸到葡萄牙的富含冰筏碎片（IRD）的沉积物组成。海因里希层代表了加拿大北部哈德逊海峡地区劳伦泰德冰盖汹涌而出的大量冰山。虽然海因里希事件已经在最后一个冰期进行了研究，但对更新世更老的冰期的海因里希事件知之甚少。在北大西洋中部U1308站点，我们发现第一个海因里希层发生在650 ka，与100 kyr周期的开始一致。然而，目前尚不清楚这是否代表了通过哈德逊海峡冰流的劳伦泰德冰盖的涌动的开始，或者这是由这一过程产生的冰山第一次在运输到U1308地点后幸存下来。前一种解释意味着在MPT前后Laurentide冰盖动力学发生了根本变化，而后一种解释表明大气和/或地表水条件的变化改变了冰山融化的位置。海因里希事件是仅限于100年的世界，还是也发生在MPT之前？我们将通过检查位于U1308站点北部和西部的站点U1304来解决这个问题。如果海因里希层是在MPT之前产生的，那么U1304站点应该记录海因里希层，因为IRD带可能在冰盖较小和北大西洋海温比晚更新世冰期更温暖的时候迁移到更高的纬度。

Earth's climate for the last 2.5 million years has been characterized by the waxing and waning of large continental ice sheets in the Northern Hemisphere. Prior to about a million years ago, the glacial and interglacial cycles marched to the beat of the tilt of Earth's rotational axis (i.e., obliquity), which varied with a cycle of 41,000 years. About a million years ago, the length of glacial-interglacial cycles became longer, averaging about 100,000 years. This change, known as the Middle Pleistocene Transition (MPT), represents an important unsolved problem in paleoclimatology. Why did the frequency of glacial-interglacial cycles suddenly shift in the middle Pleistocene from 41- to 100-kyrs? The size of the ice sheets became larger across the MPT, thereby allowing individual domes of the North American ice sheet to coalesce to form a massive ice sheet. The great volume and thickness of this ice made it less sensitive to changes in boreal summer insolation, thereby lengthening the glacial cycle as the ice sheets survived through insolation maxima and deglaciation began skipping precessional (~21 kyr) and/or obliquity (~41 kyr) cycles. One of the hindrances to furthering our understanding of the MPT is the lack of a quantitative estimate of the change in ice volume that occurred across the MPT. This shortcoming results largely because the benthic oxygen isotope record reflects both temperature and ice volume changes. Using Mg/Ca of infaunal benthic species, we aim to deconvolve the benthic d18O signal and provide a quantitative estimate of changes in the d18O of seawater that is related to global ice volume. As the North American Ice Sheet increased in volume and thickness following the MPT, the dynamical behavior of the ice sheet may have changed. During the last glacial period, Laurentide Ice Sheet dynamics was dramatically expressed by so-called Heinrich layers, which consist of sediment rich in ice rafted debris (IRD) deposited in the North Atlantic stretching from Labrador to Portugal. Heinrich layers represent massive discharges of icebergs from surging of the Laurentide Ice Sheet in the region of Hudson Strait in northern Canada. Although Heinrich events have been studied for the last glacial cycle, little is known about their occurrence in older glaciations of the Pleistocene. At Site U1308 in the central North Atlantic, we found that the first Heinrich layer occurred at 650 ka coinciding with the onset of the 100-kyr cycle. It is not known, however, whether this represented the initiation of surging of the Laurentide Ice Sheet via the Hudson Strait Ice Stream or was it the first time icebergs produced by this process survived the transport to Site U1308. The former interpretation implies a fundamental change in Laurentide ice sheet dynamics before and after the MPT, whereas the latter interpretation indicates a change in atmospheric and/oror surface water conditions that changed the location of iceberg melting. Were Heinrich events limited to the 100-kyr world or did they also occur prior to the MPT? We will address this question by examining Site U1304 located to the north and west of Site U1308. Site U1304 should record Heinrich layers if they were produced prior to the MPT because the IRD belt was probably displaced to higher latitude when ice sheets were smaller and North Atlantic SST was warmer than late Pleistocene glaciations.