有孔虫方解石壳体化学组成的生命效应机理探究

Mechanism of Vital Effect of Chemical Composition of Foraminifera Calcite Shell

有孔虫壳体钙化形成的方解石中的元素和同位素组成在重建古海洋演化方面有着广泛的应用。然而,前人发现有孔虫方解石和同海水成分的溶液中沉淀的无机方解石的化学组成有着显著差异,表明有孔虫方解石壳体的化学组成受到了“生命效应”的影响。为了探究这种“生命效应”背后的控制因素,更可靠地利用古海洋指标,前人从钙化过程及元素分配机理方面开展了一系列研究。文章总结了目前提出的有孔虫钙化的两种主要方式,即海水液泡内吞作用与Ca^(2+)离子跨膜转运作用。其中,在封闭或半封闭的钙化空间内,流体微量元素组成随着碳酸钙沉淀产生的瑞利分馏效应,成为解释有孔虫微量元素分配“生命效应”机理的经典模型。然而,这一模型仍难以定量解释有孔虫方解石壳体低镁的机制、钙同位素组成、以及镁钙比值对温度的敏感性。生物碳酸盐,包括有孔虫方解石,在形成时可能普遍存在亚稳态前驱碳酸盐物相。亚稳态前驱碳酸盐转化成方解石时产生的微量元素分配效应可能是解释有孔虫低镁的重要原因。最近,球文石物相在活体有孔虫中的发现支持了这一认识,但碳酸盐前驱体转化为方解石时的微量元素分配和同位素效应目前罕有研究。文章着重介绍了有孔虫中球文石前驱体转化为方解石时微量元素的分配模型,这一模型可以定量解释有孔虫低镁的现象。同时,结合前人实验室合成球文石和有孔虫方解石壳体的Ca同位素数据,类比颗石藻钙化过程中的Ca同位素分馏机制,作者从钙同位素角度进一步推测有孔虫方解石壳体化学组成的生命效应机理。亚稳态前驱体在有孔虫钙化过程中可能对微量元素分配与同位素分馏的“生命效应”有着重要贡献,未来需从其它微量元素和同位素的角度对该模型进一步验证。

The element and isotope compositions of calcite formed by calcification of foraminifera shells are widely used in reconstructing paleo-marine evolution.However,it has been found that the chemical composition of foraminiferal calcite is significantly different from that of inorganic calcite precipitated directly in seawater,indicating that the chemical composition of foraminiferal shell is affected by the“vital effect”.In order to explore the controlling factors behind this“vital effect”and get more reliable paleo-oceanic indicators,a series of studies have been carried out from the aspects of calcification process and element partition mechanism.This study first summarized the two main ways of foraminifera calcification:seawater vacuolization model and Ca^(2+)transmembrane model.In the enclosed or semi-enclosed calcified space,the composition of trace elements in the fluid produces a Rayleigh fractionation effect with the precipitation of calcium carbonate,which becomes a classical model to explain the mechanism of the“vital effect”of trace element partition in foraminifera.However,this model is still difficult to explain quantitatively the mechanism of low magnesium in foraminifera calcite shells,the composition of calcium isotopes,and the sensitivity of Mg/Ca ratios to temperature.Bio-carbonates,including foraminiferal calcite,may be common exist metastable precursor carbonate.The partition effect of trace elements during the conversion of metastable precursor carbonates to calcite may be an important reason to the formation of low magnesium in foraminifera.The recent discovery of vaterite in living foraminifera supports this hypothesis,but the partition of trace elements and isotope effects during the conversion of carbonate precursors to calcite are rarely studied.This paper focuses on the partition model of trace elements when the precursor of vaterite in foraminifera transformed to calcite.This model can explain the phenomenon of low magnesium in foraminifera quantitatively.At the same time,combined with the calcium isotope data synthesized in the laboratory and the calcium isotope fractionation mechanism in the calcification process of coccolith,this paper attempts to further conjecture the mechanism of“vital effect”of the chemical composition of foraminifera calcite shells from the perspective of calcium isotope.Metastable precursors may contribute significantly to the“vital effect”of trace elements partition and isotope fractionation during foraminifera calcification,and this model needs to be further verified from the perspective of other trace elements and isotopes.