A critical assessment of mollusk 18 O/16 O, Mg/Ca, and 44 Ca/40 Ca ratios as proxies for Cretaceous seawater temperature seasonality
A critical assessment of mollusk 18 O/16 O, Mg/Ca, and 44 Ca/40 Ca ratios as proxies for Cretaceous seawater temperature seasonality
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DOI：
10.1016/j.palaeo.2004.09.005
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发表时间：
2005-01
期刊：
影响因子：
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通讯作者：
A. Immenhauser;T. Nägler;T. Steuber;D. Hippler
中科院分区：
文献类型：
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作者：
A. Immenhauser;T. Nägler;T. Steuber;D. Hippler
特提斯沿海水域的季节性温度波动记录在一个来自土耳其的原始的，大约80万年的rudist贝壳中。为了批判性地评价海水温度以外可能影响壳类地球化学记录的因素，采用了3种不同的古温度代用指标（δ18O、Mg/Ca和δ44Ca）。在定性方面，所有三个代用指标反映了相同的周期性趋势，从而为晚白垩世表层海水温度的季节性波动提供了有力的证据。这表明新的δ44Ca温度代用方法在软体动物方解石化石上的成功应用。然而，三个数据集的直接比较表明，所有代理都充满了问题。与其他研究类似，18O/16O比率表明，与今天可比的沿海环境相比，夏季最高温度更高。然而，18o /16O代理受制于可能导致峰值温度高估的环境因素。此外，Mg/Ca摩尔比对环境因素的敏感性较低，但受到双壳贝对白垩纪海水低Mg/Ca比的离子调节能力的强烈影响。同样，白垩纪海水δ44Ca组成的不确定性和软体动物复杂的生物钙化，目前限制了用海水绝对温度来解释δ44Ca值。然而，这里采用的多代理方法证明，这些限制并没有掩盖生物地球化学地层档案中记录的季节性海面温度变化的主要模式。
Seasonal temperature fluctuations of Tethyan coastal water are recorded in a pristine, ∼80-My-old rudist shell from Turkey. In order to critically assess factors other than seawater temperature that might have influenced the shell geochemical record, three different paleo-temperature proxies (δ18O, Mg/Ca and δ44Ca) were applied. In a qualitative manner, all three proxies reflect the same cyclical trends and thus yield robust evidence for seasonal fluctuations in Late Cretaceous surficial seawater temperature. This suggests the successful application of the new δ44Ca temperature proxy to fossil mollusk calcite. The direct comparison of the three data sets, however, demonstrates that all proxies are fraught with problems. Similar to other studies,18O/16O ratios point to warmer summer temperature maxima than those in comparable costal settings today. Nevertheless, the18O/16O proxy is subject to environmental factors that might lead to an overestimation of peak temperatures. Moreover, Mg/Ca molar ratios are less sensitive to environmental factors but were strongly affected by the ion regulating capability of the rudist bivalve that responded to the low Mg/Ca ratio of Cretaceous seawater. Similarly, uncertainties of the δ44Ca composition of Cretaceous seawater and the complex bio-calcification of mollusks presently limit the interpretation of δ44Ca values in terms of absolute seawater temperature. The multiproxy approach applied here, however, documents that these limitations do not obscure the dominant patterns of seasonal sea-surface temperature variations recorded in the biogeochemical rudist archive.