Improved chronostratigraphic reference curve of late Neogene seawater 87 Sr / 86 Sr : Comment and Reply COMMENT
Improved chronostratigraphic reference curve of late Neogene seawater 87 Sr / 86 Sr : Comment and Reply COMMENT
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改进的新近纪晚期海水年代地层参考曲线 87 Sr / 86 Sr : 评论和回复 COMMENT
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发表时间：
1996
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通讯作者：
G. Friedman
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作者：
G. Friedman
我的评论的目的是警告其他研究人员，与公海海水的Sr/Sr无关的因素可能决定碳酸盐矿床的锶同位素比率。这个警告是必要的，因为Farrell等人（1995）的参考曲线的潜在用户可能会错误地解释他们样本的年龄。在我对西奈亚板块现代碳酸盐沉积物的研究中（Friedman, 1995），锶同位素的变化与Farrell等人（1995）得出的数据不一致。在胶结基质中的甘草酸壳形成一个连珠。虽然它们的锶同位素比值0.709137（11）略低于Farrell et al.（1995）曲线，但仍与现代海水的同位素比值一致。然而，壳间基质的锶同位素比值为0.708988(13)，放射性碳年代为6300年，与Burke等人（1982）图中新世-上新世边界附近的海水同位素比值相当，与Farrell等人（1995）曲线上约5 ~ 5.5 Ma的海水同位素比值相当（图1）。现代高盐池碳酸盐岩样品的同位素组成也表现出与海水的不平衡。它们的锶同位素比值分别为0.708043（14）和0.707862（14）。在Farrell等人（1995）的图表上，这些现代碳酸盐沉积物无法绘制，因为它们低于他们的所有数据，而在Burke等人（1982）的图表上，显示了中新世中期。然而放射性碳年代在26至1050年之间（Friedman, 1995）。来自现代sabkha的样品显示碳酸盐沉积物的锶同位素组成与白垩纪海水的锶同位素组成0.70756（10）一致，但其放射性碳年龄为11190 6 290 C-14 yr B.P. （C-13校正）。我取样的现代碳酸盐沉积物来自地中海和红海的不同环境，包括死海在分隔阿拉伯板块和西奈板块的板块边界处的转变。这些现代碳酸盐相中的锶同位素比率在降水后发生了变化，或者它们是从比现代海水更低放射性的流体中沉淀下来的。低锶含量可能与压实过程中从深处排出的古海水有关，也可能与来自地幔的流体有关，或者与更古老的基岩的再循环有关。使用Farrell等人（1995）提出的“年代地层参考曲线”的人可能希望确保他们的样品与海水Sr/Sr严格相关。然而，地质变量有时甚至比地质学家本身复杂得多
The purpose of my Comment is to warn other researchers that factors unrelated to Sr/Sr of open-ocean sea water may determine the strontium isotopic ratio of carbonate deposits. This cautionary note is necessary because potential users of Farrell et al.’s (1995) reference curve may interpret incorrectly the ages of their samples. In my work on modern carbonate sediments of the Sinai subplate (Friedman, 1995), strontium isotopic variation is at variance with the data that Farrell et al. (1995) have generated. Glycymeris shells in a cemented matrix form a coquinite. Although their strontium isotopic ratio of 0.709137 (11) falls slightly below the Farrell et al. (1995) curve, it is still consistent with that of modern sea water. The strontium isotopic ratio of the matrix between the shells at 0.708988 (13), however, dated by radiocarbon as 6300 yr B.P., is equivalent to seawater ratios near the Miocene-Pliocene boundary of Burke et al.’s (1982) graph and approximately 5 to 5.5 Ma on the curve of Farrell et al. (1995) (Fig. 1). The isotopic composition of modern hypersaline pool carbonate samples shows likewise disequilibrium with respect to sea water. Their strontium isotopic ratios are 0.708043 (14) and 0.707862 (14). On the Farrell et al. (1995) graph, these modern carbonate sediments cannot be plotted because they fall below all their data, and on the Burke et al. (1982) graph a mid-Miocene age is indicated. Yet radiocarbon dates range between 26 and 1050 yr B.P. (Friedman, 1995). A sample from a modern sabkha showed the strontium isotopic composition of carbonate sediment to be consistent with that of Cretaceous seawater 0.70756 (10), yet its radiocarbon age gave 11,190 6 290 C-14 yr B.P. (C-13 corrected). The modern carbonate sediments that I have sampled are from various settings of the Mediterranean and Red Sea, including the Dead Sea transform at the plate boundary that separates the Arabian from the Sinai plates. Either strontium isotopic ratios in these modem carbonate facies have changed since precipitation, or they precipitated from a less radiogenic fluid than modern sea water. Perhaps the low strontium numbers relate to paleoseawater expelled from depth during compaction, to fluids derived from the mantle, or to recycling from older bedrock. Users of the ‘‘chronostratigraphic reference curve’’ presented by Farrell et al. (1995) may wish to assure themselves that their samples relate strictly to seawater Sr/Sr. Yet geologic variables are far more complex than even geologists themselves sometimes