Evolution of the ocean's “biological pump”
Evolution of the ocean's “biological pump”
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DOI：
10.1073/pnas.1112236108
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发表时间：
2011-09
期刊：
影响因子：
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通讯作者：
A. Ridgwell
中科院分区：
文献类型：
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作者：
A. Ridgwell
地球历史被气候和全球生物地球化学循环中各种各样的转变和扰动所打断。这些可能与重大物种灭绝或进化创新有关，可能显示温室气体变暖和二氧化碳排放的证据，因此可能持有与未来相关的直接信息(1)或可能与冰河时代有关。可以说，没有什么事件比新元古代(1,000-542 Ma)(2)期间的极端冰川发生更神秘，也没有比这更引起激烈争论的了(2)，当时，全球海洋可能在两个不同的时期达到完全的海冰覆盖，形成一个“雪球地球”(3)。完全理解这些强烈冰川事件的主要障碍之一是确定触发因素；因为在之前约1,500 Ma或之后显生宙期间的任何时间都没有发生类似的事件。初始冷却的机制也必须与海洋碳酸盐碳同位素组成(即δ13C)中记录的显著负漂移一致(2)。后一种标准特别具有挑战性，因为尽管有机物质和水合物甲烷等还原生物形式的碳的氧化在13C中高度消耗，因此能够使海洋和大气的δ13C为负值，但二氧化碳(和甲烷)等温室气体的排放往往会防止而不是导致深冰期(4)。在《美国国家科学院院刊》上，齐珀曼和他的同事(5)通过认识到氧化有机物的替代途径很重要，以及海洋中的碳循环可能在这一时期发生了深刻的变化，从而在冷却[和大气中较低的二氧化碳分压]和负趋势δ13C之间划出了圆圈。
Earth history is punctuated by a huge variety of transitions and perturbations in climate and global biogeochemical cycles. These may be linked to major extinctions or evolutionary innovations, and may exhibit evidence for greenhouse warming and CO2 release and hence potentially hold direct future-relevant information (1) or may be associated with ice ages. Arguably, no event is more enigmatic or has been more keenly debated than the occurrence of extreme glaciation during the Neoproterozoic (1,000–542 Ma) (2), when, in two separate episodes, the global ocean potentially attained complete sea-ice cover to create a “snowball Earth” (3). One of the main barriers to a full understanding of these intense glacial episodes has been in identifying the trigger; as nothing comparable occurs at any time in the preceding approximately 1,500 Ma or afterward during the Phanerozoic. The mechanism for the initial cooling must also be consistent with a pronounced negative excursion recorded in the carbon isotopic composition (i.e., δ13C) of marine carbonates (2). This latter criterion is particularly challenging because, although the oxidation of reduced, biological forms of carbon such as organic matter and hydrate methane are highly depleted in 13C and hence are able to drive ocean and atmosphere δ13C negative, the release of greenhouse gases such as CO2 (and CH4) will tend to prevent, not cause, a deep ice age (4). In PNAS, Tziperman and colleagues (5) square the circle between cooling [and a lower partial pressure of CO2 (pCO2) in the atmosphere] and negative-trending δ13C by recognizing that alternative pathways for oxidizing organic matter would have been important and that carbon cycling in the ocean may have undergone profound changes around this time.