Stable isotope fractionation in live benthic foraminifera from the southern California Borderland
Stable isotope fractionation in live benthic foraminifera from the southern California Borderland
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南加州边境活底栖有孔虫的稳定同位素分馏
DOI：
10.1016/0031-0182(84)90100-7
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发表时间：
1984
期刊：
影响因子：
--
通讯作者：
E. Grossman
中科院分区：
文献类型：
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作者：
E. Grossman
从南加州边界108-1134米深度的表层沉积物样本中分析了底栖有孔虫的活体标本。还测量了环境水和溶解无机碳的温度和同位素组成，以便将有孔虫组成与预期的平衡值进行比较。分析的物种分为四类，反映了矿物学(文石与方解石)、生物分馏(生命效应)和微生境效应对同位素组成的影响。文石相Hoeglundina eleans18O相对平衡方解石在浅水中的富集量为0.41%0，在深水中至少增加到1.06%0。~(13)C的富集度。相对溶解无机碳(DIC)的含量也与深度有关，从108m时的1.27%0增加到940 m时的2.13%0，这种深度依赖关系是DIC组成变化的结果；由于某些未知的原因，优雅保持不变的深度。一种可能的解释是，随着温度的降低，Hoeglundina-Hco−3分馏增加平衡了DIC组成的变化。Cassidulinas种的δ13C值与环境溶解无机碳的值几乎相同，反映了DIC的δ13C随深度的减少。未处于~(18)O平衡状态的物种的δ~(13)C值与DIC组成的变化无关，δ~(13)C和δ~(18)O在同年代物种中存在相关性，这与解释同位素轻质代谢CO_2并入壳层的生命效应的模型一致。18O的不平衡程度相对恒定，但13C的不平衡程度不变，这表明在同位素地层学研究中，可能很难像δ180所做的那样“校准”不同物种的δ13C。分类群和生命效应之间似乎存在关系。与水的平衡约为18O的有孔虫，包括Cassidulina、Borvina、Uvilerina和GloboBulimina的物种，是Cassidulinidae或Buliminacea超科的成员。未达到~(18)O平衡的种类，包括金龟子、五眼金龟子、三眼金龟子和豆角豆科，属于蝇科和天牛科。在边疆的低氧环境中尚未发现不平衡物种。这一观察和发表的关于底栖有孔虫超微结构的研究结果表明，同位素“行为”可能与有孔虫与周围水快速交换气体的能力有关。
Live specimens of benthic foraminifera have been analyzed from surface sediment samples collected from 108 to 1134 m depth in the Southern California Borderland. The temperature and isotopic composition of the ambient water and dissolved inorganic carbon have also been measured to permit comparison of foraminiferal compositions to expected equilibrium values.The species analyzed break down into four groups which reflect the influence of mineralogy (aragonite vs. calcite), biological fractionation (“vital effect”), and microhabitat effects on isotopic composition. The aragoniticHoeglundina elegansshow an enrichment in18O relative to equilibrium calcite of 0.41%0in shallower water increasing to at least 1.06%0in deeper water. The13C enrichment ofH. elegansrelative to the dissolved inorganic carbon (DIC) is also depth dependent, increasing from 1.27%0at 108 m to 2.13%0at 940 m. This depth dependence results from the change in composition of the DIC; the13C composition ofH. elegansremains constant with depth for some unknown reason. One possible explanation is that the change in composition of the DIC is balanced by an increase inHoeglundina—HCO−3fractionation with decreased temperature.The δ13C values of theCassidulinaspecies are almost identical to those of the ambient dissolved inorganic carbon (DIC) and reflect the decrease in the δ13C of the DIC with depth. The δ13C values of species not in18O equilibrium vary independently of the changing composition of the DIC.There is a correlation between δ13C and δ18O among coeval species, consistent with the model which explains vital effect as the result of incorporation of isotopically light metabolic CO2into the shell. The magnitude of disequilibrium is relatively constant for18O but not for13C, suggesting that in isotopic stratigraphy studies, it may be difficult to “calibrate” the δ13C of different species as has been done with δ18O.There appears to be a relationship between taxonomic group and vital effect. The foraminifera in approximate18O equilibrium with the water, including species ofCassidulina, Bolivina, Uvigerina, andGlobobulimina, are members of the family Cassidulinidae or superfamily Buliminacea. Those not in18O equilibrium, including species ofPyrgo, Quinqueloculina, Triloculina, andLenticulina, belong to the families Miliolidae and Vaginulinidae.No disequilibrium species have been found as yet in the low oxygen environments of the borderland. This observation and published findings on the ultrastructure of benthic foraminifera suggest that isotopic “behavior” may be related to a foraminifer's ability to exchange gases rapidly with its ambient water.