IODP Exp. 362 - Carbonate accumulation events in the Eocene Indian Ocean

始新世，5600万至3400万年前，比今天高出约14摄氏度，南极洲或格陵兰岛没有冰盖，大气二氧化碳浓度在500至1500 ppm之间(而公元1750年前的水平约为280 ppm)。在过去，始新世的结束(约3390万年前)被认为是南极主要冰盖的第一次出现，然而，最近的研究表明，始新世中晚期的气候可能非常动态，包括南极洲的大量冰增长，随后在始新世结束时，随着大气二氧化碳浓度的大幅下降，南极洲形成了更大、更永久的冰帽。计算机模型表明，碳酸钙(形成海洋浮游生物外壳，沉入海底形成沉积物)在海洋中溶解的深度(碳酸钙补偿深度，ccd)与海洋酸度密切相关，而海洋酸度又与大气中的二氧化碳有关。尽管过去二氧化碳浓度的可靠记录仍然很少，但研究人员已经确定了始新世中后期太平洋CCD值的显著波动(导致沉积碳酸盐堆积事件或CAES)，这可能与大气二氧化碳浓度的变化有关。在现代海洋中，印度洋和太平洋的电荷耦合器件的深度相似。然而，电荷耦合器件的深度也取决于深海的温度和压力，以及控制海洋盆地深水年龄的海洋环流--这一点很重要，因为海水中溶解的二氧化碳浓度自上次暴露在大气中以来随着时间的推移而增加。尽管研究人员认为CAES很可能是全球性事件，但尚未有来自印度洋的研究证实那里的CCD值以类似的方式变化，那里的始新世中期和晚期的海洋条件可能有很大的不同。这一建议基于从东赤道印度洋IODP站点U1480(水深4148米)海底收集的沉积物岩芯。中、晚始新世沉积剖面含有钙质软泥条带，与深色富含粘土的沉积物相间。我们认为，这些钙质带代表了至少与部分太平洋CAE相关的CCD值的加深，这可能证实了这些事件的全球性。为了验证这一全球假设，我们建议生成沉积碳酸钙含量的记录，其中通常使用10%的浓度阈值来定义电荷耦合器件的位置。为了最好地描述沉积体制的巨大变化，从而保存了引人注目的亮带和暗带，我们还建议使用高分辨率分析扫描电子显微镜来分析沉积物。我们将记录从浅层到暗层过渡过程中沉积物组构和粒度的变化，并制作相应的高分辨率化学元素地图。在这种微尺度上对沉积物进行分析，将使我们能够利用微化石和沉积物地球化学来解释什么样的古环境变化与沉积体制的如此剧烈波动有关。此外，我们将调查轻钙质渗流带中的毫米级纹层是否与任何可能与来自南大洋的深水变化有关的颗粒尺寸变化有关，这些变化可能在较凉爽的气候时期被激活。

The Eocene period, 56-34 million years ago, was around 14 degrees Celsius warmer than today with no ice sheets on Antarctica or Greenland and atmospheric carbon dioxide concentrations of between 500 and 1500 ppm (compared to pre-1750 AD levels of ~280 ppm). In the past, the termination of the Eocene period (c. 33.9 million years ago) was thought to have represented the first appearance of major ice sheets on Antarctica, however, recent research has suggested that middle/late Eocene climate may have been very dynamic including substantial ice growth on Antarctica, followed by larger, more permanent ice caps developing on Antarctica at the end of the Eocene at the same time as a major decrease in atmospheric carbon dioxide concentrations. Computer models suggest that the depth at which calcium carbonate (which forms the shells of the marine plankton that sink to the ocean floor to form sediment) dissolves in the ocean (the calcium carbonate compensation depth, CCD) is closely linked to ocean acidity, which in turn is linked to atmospheric crbon dioxide. While reliable records of past-carbon dioxide concentrations remain scarce, researchers have identified significant fluctuations of the Pacific CCD during the middle/late Eocene (leading to sedimentary carbonate accumulation events or CAEs), which were likely associated with changes in atmospheric carbon dioxide concentrations. In the modern ocean, the depth of the CCD is similar in the Indian and Pacific Oceans. However, the depth of the CCD is also dependent on the temperature and pressure of the deep ocean, and on oceanic circulation which controls the age of deep waters in ocean basins - important because the concentration of dissolved carbon dioxide in sea water increases with the time since it was last exposed to the atmosphere. Whilst researchers think CAEs were likely global events, no studies from the Indian Ocean have yet confirmed that the CCD varied in a similar way there, where middle/late Eocene oceanic conditions may have been significantly different. This proposal is based on sediment cores collected from the sea floor at IODP Site U1480 in the eastern equatorial Indian Ocean (4148 m water depth). The middle/late Eocene sediment section contained bands of calcareous ooze alternating with darker clay-rich sediments. We propose that these calcareous bands represent deepening of the CCD that is correlated with at least some of the Pacific CAEs, which could confirm the global nature of these events. In order to test this global hypothesis, we propose generating a record of sedimentary calcium carbonate content, where a <10% concentration threshold is usually used to define the position of the CCD. In order the best characterise the dramatic changes in sedimentary regime that lead to the preservation of the striking light and dark banding, we also propose to analyse the sediments using a high-resolution analytical scanning electron microscope. We will document changes in sediment fabrics and grainsize across the transitions from light to dark layers and produce accompanying high-resolution chemcial element maps. Analysing at the sediment at this microscale will allow us to interpret what kind of palaeoenvironmental changes were associated with such dramatic fluctuations of the sedimentary regime, using microfossils and sediment geochemistry. Further, we will investigate whether millimeter-scale laminations in the light calcareous ooze bands are associated with any changes in grain size that could be associated with variations in deep water from the Southern Ocean, which was likely invigorated during cooler climate periods.