Coupled change in global climate and the carbon cycle across the Eocene-Oligocene transition: New insight from the Pacific Ocean, IODP Exp 320

二氧化碳是一种强大的温室气体，自工业革命开始以来（大约250年的时间内），其在地球大气中的浓度增加了约35%，达到了以冰芯气泡测量的过去80万年中任何时候都要高的水平。如果人为（人为）向大气排放的二氧化碳按照预计的速率，那么到2100年，二氧化碳浓度将达到古近纪（约6500万至2300万年前）以来地球上从未见过的水平。这些惊人的观察结果意味着我们必须提高对古近纪气候的认识。可以说，关键的古近纪气候事件发生在34 Ma左右，当时第一个大型南极冰盖在始新世/渐新世过渡时期迅速建立。冰盖气候模式实验的结果表明，这一事件可能代表了对大气二氧化碳水平缓慢下降（比人为增加的速度慢许多数量级）的“临界点”反应。我们自己先前的研究结果表明，南极冰盖在EOT上的增长与热带太平洋方解石补偿深度（CCD）的加深（超过一公里）是同步的。CCD是碳酸钙沉积物在海洋中溶解的深度，可以被认为是“海洋酸度指标”。整个EOT的海洋去酸化表明，从一个基本上没有冰川的“温室”世界到南极洲有大片冰盖的“温室”世界的转变与全球碳循环的严重中断密切相关。是什么将南极洲冰盖的增长与热带太平洋深处的酸度联系起来？以前我们曾提出，南极大陆冰盖的增长导致海平面下降，杀死了大量碳酸钙（CaCO3）——以前在大陆架上繁盛的分泌碳酸钙的珊瑚礁。这将导致CaCO3向全球海洋的输入（来自河流）和输出（沉积物掩埋）之间的不平衡，需要加深CCD，直到通过增加深海CaCO3沉积恢复平衡。在数值“盒子模型”测试中，这种“大陆架到盆地CaCO3切换”机制比解释EOT事件的竞争机制表现得更好。然而，大陆架-盆地转换在许多方面都受到质疑。例如，有人提出，整个东太平洋的碳循环变化是由表层海洋生产力的变化驱动的。2009年，综合海洋钻探计划远征320队在太平洋上不同年龄的海洋地壳上钻了一系列孔。EOT是在连续较浅深度的四个新地点捕获的。我们的计划是将新地点的EOT“年龄深度样带”的化学成分与我们之前在该地区钻探的地点的纬度样带的现有记录进行比较（ODP Leg 199）。EOT的假设是，为了响应全球深海碳酸盐化学的变化而调用CCD加深。陆架-盆地转换)应该产生一种沉积模式，这是深度的简单函数，而我们通过地表生产力的变化来预测纬度的印记。我们工作的一个关键是在我们的目标沉积物中进行前所未有的年龄控制。其他关键方面包括两个新的地点的可用性，这些地点比以前最好的太平洋地点（ODP 1218）更富含CaCO3和钙质微化石，这些地点横跨EOT（特别是最晚的始新世），以及用于确定古代海洋碳酸盐化学变化的新的地球化学技术。

Carbon dioxide, CO2, is a powerful greenhouse gas and its concentration in Earth's atmosphere has increased by around 35% since the start of the Industrial Revolution (in a ca. 250 yr time period) to a level that is higher than at any time in the past 800 thousand years as measured in air bubbles from ice cores. If man-made (anthropogenic) CO2 emissions to the atmosphere follow projected rates, then by 2100, concentrations will reach values not seen on Earth since the Palaeogene epoch (ca. 65-23 million years ago, Ma). These startling observations mean that we must improve our understanding of Palaeogene climate. Arguably, the pivotal Palaeogene climate event occurred around 34 Ma when, the first large Antarctic ice sheets were rapidly established across the Eocene/Oligocene transition, EOT. Results of ice-sheet-climate model experiments indicate that this event might represent a 'tipping point' response to slow decline in atmospheric CO2 levels (many orders of magnitude slower than the rate of anthropogenic increase). The results of our own previous research show that the growth of ice sheets on Antarctica across the EOT occurred in lock-step with a deepening (by more than a kilometer) in the calcite compensation depth (CCD) in the tropical Pacific Ocean. The CCD is the depth at which calcium carbonate sediments are dissolved in the ocean and can be thought of as 'an ocean acidity indicator'. Ocean de-acidification across the EOT demonstrates that the switch from a largely non-glaciated 'greenhouse' world to one with large ice sheets on Antarctica was closely associated with a big disruption in the global carbon cycle. What linked ice sheet growth on Antarctica to the acidity of the tropical deep Pacific Ocean? Previously we have suggested that growth of ice sheets on the Antarctic continent causes sea level to fall, killing off the large expanse of calcium carbonate (CaCO3)-secreting reefs that previously flourished on the continental shelves. This would have caused an imbalance between the inputs (from rivers) and outputs (sediment burial) of dissolved CaCO3 to the global ocean requiring a deepening in the CCD until balance was restored through increased CaCO3 sedimentation in the deep ocean. In numerical 'box model' tests, this 'shelf-to-basin CaCO3 switching' mechanism performed better than competing mechanisms proposed to explain EOT events. Yet shelf-basin-switching has been called into question on a number of grounds. For example, it has been suggested that the carbon cycle changes across the EOT were driven by changes in surface ocean productivity. In 2009, Integrated Ocean Drilling Program Expedition 320 drilled a series of holes across the Pacific Ocean on ocean crust of different age. The EOT was captured in four new sites at successively shallower depths. Our plan is to compare the chemistry of this EOT 'age-depth transect' of new sites with our existing records from a latitudinal transect of sites drilled previously in the region (ODP Leg 199). Hypotheses for the EOT that invoke CCD deepening in response to changes in global deep ocean carbonate chemistry (eg. shelf-basin-switching) should produce a pattern of sedimentation that is a simple function of depth whereas we predict the imprint of latitude from changes in surface productivity. One key to our work is the potential for unprecedented age-control in our target sediments. Other key aspects are the availability of two new sites that are richer in CaCO3 and calcareous microfossils across the EOT (specifically the latest Eocene) than the best previously available Pacific site (ODP 1218) and new geochemical techniques for determining changes in carbonate chemistry of the ancient oceans.