Geologic history of sea water
Geologic history of sea water
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海水地质历史
DOI：
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发表时间：
1961
期刊：
影响因子：
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通讯作者：
William Walden Rubey
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文献类型：
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作者：
William Walden Rubey
古生物学和生物化学最终可能会产生有关海水和大气古化学的相当确定的信息。现在已有的几条不太确定的证据表明，海水和大气的成分在过去可能有所变化;但地质记录表明，这些变化可能在相对狭窄的范围内。一个主要的问题是，条件如何能在如此长的时间内保持如此接近恒定。即使从古代沉积物的数量和成分的不充分数据中也可以清楚地看出，在现在的大气圈、水圈、生物圈和古代沉积物中，更易挥发的物质--H2O、CO2、Cl、N和S--太丰富了，就像更常见的造岩氧化物一样，不能仅仅解释为岩石风化的产物。如果地球曾经完全是气态或熔融态的，那么这些“多余”的挥发物可能是原始大气层的残余物。但如果是这样的话，那么就应该得出一些推论，即溶解在熔融地球中的水的数量，以及高酸性原始海洋的预期化学效应。这些推论似乎与地质记录相矛盾，因此，人们对致密的原始大气的假设产生了怀疑。似乎更有可能的是，在早期的大气和海洋中，只有一小部分的“过量”挥发物曾经同时存在。碳在海水化学和生物领域中起着重要作用。现在沉积岩中以碳酸盐和有机碳的形式埋藏的碳量大约是今天大气圈、水圈和生物圈中碳量的600倍。如果只有1/100的这种埋藏的碳突然增加到现在的大气和海洋中，许多海洋生物物种可能会灭绝。此外，除非CO2从岩石风化以外的其他来源持续地增加到大气-海洋系统中，否则，以目前沉积作用减少CO2的速度，仅需几百万年，水镁石就会取代方解石成为一种常见的海洋沉积物。显然，地质记录没有证据表明许多物种同时灭绝，也没有水镁石的沉积。显然，在过去的地质年代中，大气和海洋中的二氧化碳含量一直保持相对稳定。这就要求除了岩石风化和较老的沉积岩的变质作用之外，还有某种渐进的、持续的供应来源。不同的“过量”挥发物的相对量提供了这一来源的线索。这与从火山、火山口和温泉逸出的气体以及在火成岩中封闭的气体中相同物质的相对含量相似。因此可以想象，水圈和大气层可能几乎完全来自这种深成气体。在岩浆结晶过程中，挥发物如H2O和CO2在剩余的熔体中积累，并作为最终馏分的一部分被大量排出。在过去的地质历史中，火山爆发和熔岩流将挥发物带到地球表面，但侵入岩可能是大气圈和水圈成分的更充分的来源。根据美国的温泉来判断，温泉（只携带1%或更少的年轻物质）可能是“多余的”挥发物从下面冷却的岩浆中逃逸出来的主要通道。这种机制不能解释挥发物的持续供应，除非它也提供了新的，富含挥发物的岩浆的持续生成。这种局部岩浆可能是由地壳下岩石的连续选择性熔融过程形成的，其深度在地壳较移动的区域之下几百公里。这意味着海洋的体积随着时间的推移而增加。在这一点上，地质证据允许有不同的解释;不可否认的是，记录并没有证明，但似乎与大陆块的不断增长和海洋盆地的不断下沉相一致。也许下面的机制可以解释为什么挥发物不断地逃逸到地球表面，并且在大部分地质时期海水的成分相对均匀：（1）来自不稳定大陆边缘和地槽之下的深层几乎无水岩石的低熔点部分的选择性熔融;（2）这些选定馏分的上升（3）分异大陆块体与相邻洋盆之间实质上持续的均衡调整;（4）侵蚀和沉积作用的恢复，导致大陆边缘和山区的不稳定，并导致新一轮的选择性融合。
Paleontology and biochemistry together may yield fairly definite information, eventually, about the paleochemistry of sea water and atmosphere. Several less conclusive lines of evidence now available suggest that the composition of both sea water and atmosphere may have varied somewhat during the past; but the geologic record indicates that these variations have probably been within relatively narrow limits. A primary problem is how conditions could have remained so nearly constant for so long. It is clear, even from inadequate data on the quantities and compositions of ancient sediments, that the more volatile materials—H 2 O, CO 2 , Cl, N, and S— are much too abundant in the present atmosphere, hydrosphere, and biosphere and in ancient sediments to be explained, like the commoner rock-forming oxides, as the products of rock weathering alone. If the earth were once entirely gaseous or molten, these “excess” volatiles may be residual from a primitive atmosphere. But if so, certain corollaries should follow about the quantity of water dissolved in the molten earth and the expected chemical effects of a highly acid, primitive ocean. These corollaries appear to be contradicted by the geologic record, and doubt is therefore cast on this hypothesis of a dense primitive atmosphere. It seems more probable that only a small fraction of the total “excess” volatiles was ever present at one time in the early atmosphere and ocean. Carbon plays a significant part in the chemistry of sea water and in the realm of living matter. The amount now buried as carbonates and organic carbon in sedimentary rocks is about 600 times as great as that in today9s atmosphere, hydrosphere, and biosphere. If only 1/100 of this buried carbon were suddenly added to the present atmosphere and ocean, many species of marine organisms would probably be exterminated. Furthermore, unless CO 2 is being added continuously to the atmosphere-ocean system from some source other than rock weathering, the present rate of its subtraction by sedimentation would, in only a few million years, cause brucite to take the place of calcite as a common marine sediment. Apparently, the geologic record shows no evidence of such simultaneous extinctions of many species nor such deposits of brucite. Evidently the amount of CO 2 in the atmosphere and ocean has remained relatively constant throughout much of the geologic past. This calls for some source of gradual and continuous supply, over and above that from rock weathering and from the metamorphism of older sedimentary rocks. A clue to this source is afforded by the relative amounts of the different “excess” volatiles. These are similar to the relative amounts of the same materials in gases escaping from volcanoes, fumaroles, and hot springs and in gases occluded in igneous rocks. Conceivably, therefore, the hydrosphere and atmosphere may have come almost entirely from such plutonic gases. During the crystallization of magmas, volatiles such as H 2 O and CO 2 accumulate in the remaining melt and are largely expelled as part of the final fractions. Volcanic eruptions and lava flows have brought volatiles to the earth9s surface throughout the geologic past; but intrusive rocks are probably a much more adequate source of the constituents of the atmosphere and hydrosphere. Judged by the thermal springs of the United States, hot springs (carrying only 1 per cent or less of juvenile matter) may be the principal channels by which the “excess” volatiles have escaped from cooling magmas below. This mechanism fails to account for a continuous supply of volatiles unless it also provides for a continuous generation of new, volatile-rich magmas. Possibly such local magmas form by a continuous process of selective fusion of subcrustal rocks, to a depth of several hundred kilometers below the more mobile areas of the crust. This would imply that the volume of the ocean has grown with time. On this point, geologic evidence permits differences of interpretation; the record admittedly does not prove, but it seems consistent with, an increasing growth of the continental masses and a progressive sinking of oceanic basins. Perhaps something like the following mechanism could account for a continuous escape of volatiles to the earth9s surface and a relatively uniform composition of sea water through much of geologic time: (1) selective fusion of lower-melting fractions from deep-seated, nearly anhydrous rocks beneath the unstable continental margins and geosynclines; (2) rise of these selected fractions (as granitic and hydrous magmas) and their slow crystallization nearer the surface; (3) essentially continuous isostatic readjustment between the differentiating continental masses and adjacent ocean basins; and (4) renewed erosion and sedimentation, with resulting instability of continental margins and mountainous areas and a new round of selective fusion below.