SR/MG RATIOS OF MODERN MARINE CALCITE - EMPIRICAL INDICATORS OF OCEAN CHEMISTRY AND PRECIPITATION RATE
SR/MG RATIOS OF MODERN MARINE CALCITE - EMPIRICAL INDICATORS OF OCEAN CHEMISTRY AND PRECIPITATION RATE
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DOI：
10.1016/0016-7037(92)90314-9
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发表时间：
1992-05-01
影响因子：
5
通讯作者：
LOHMANN, KC
中科院分区：
文献类型：
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作者：
CARPENTER, SJ;LOHMANN, KC
全新世生物方解石和非生物海相方解石的镁含量范围相似(分别为0~22和4~21摩尔%碳酸镁)，但生物方解石的Sr2+浓度始终高于非生物方解石的1250ppm。在实验室实验中，D(锶)与方解石镁含量之间存在着正的线性关系。这在Sr2+与Mg2+浓度的曲线图上产生了两个截然不同的线性趋势。这两个趋势的主变轴具有相似的斜率，但截获的Sr2+明显不同。(生物性：Y=0.024x+1298，R2=0.70；非生物性：Y=0.027x+47，R2=0.77)。这些趋势的相似斜率反映了现代海水中镁/钙和锶/钙比值的稳定性。生物的D(Sr)=3.16X10(-6)(ppm镁)+0.169；非生物的D(Sr)=3.52X10(-6)(ppm镁)+0.0062。全新世物质的锶-镁趋势特征可以与古代样品的类似趋势相比较，以估计海水镁/钙和锶/钙比值的相对变化。生物方解石中相对较高的锶含量是由于海洋生物中贝壳沉积的快速沉淀速率所致。实验室实验中从海水中沉淀的方解石的D(Sr)值与生物海相方解石的D(Sr)值相似，表明两者的沉淀速度大致相同。我们对浮游有孔虫和底栖有孔虫表面积归一化降雨率的估计与播种、pH恒定实验的结果相当。实验结果表明，生物方解石和实验海相方解石的D(Sr)值受动力学控制，而非生物海相方解石的沉淀率较低，得到的D(Sr)值接近平衡条件。描述D(Sr)与方解石沉淀率关系的实验方程表明，生物方解石和非生物方解石的锶含量偏差是非生物沉淀物的沉淀率比生物沉淀物低2~5个数量级的结果。然而，对自然形成的海洋水泥的观察表明，五个数量级的偏移量最能代表自然系统的过程。
Holocene biotic and abiotic marine calcite have a similar range of Mg contents (0 to 22 and 4 to 21 mol% MgCO3, respectively), yet biotic calcite has Sr2+ concentrations that are consistently 1250 ppm higher than those of abiotic calcite. As in laboratory experiments, a positive linear relation is observed between D(Sr) and calcite Mg content. This produces two distinct linear trends on a plot of Sr2+ vs. Mg2+ concentrations. Principal axes of variation for both trends have similar slopes, yet distinctly different Sr2+ concentration intercepts. (Biotic: y = 0.024x + 1298, r2 = 0.70; Abiotic: y = 0.027x + 47, r2 = 0.77). The similar slopes of these trends reflect the constancy of Mg/Ca and Sr/Ca ratios of modern seawater. Equations describing the dependence of D(sr) on calcite Mg content are derived from both trends (Biotic: D(sr) = 3.16 X 10(-6) (ppm Mg) + 0.169; Abiotic: D(sr) = 3.52 X 10(-6) (ppm Mg) + 0.0062). Characterization of Sr-Mg trends for Holocene materials allows comparison with analogous trends from ancient samples to estimate relative changes in seawater Mg/Ca and Sr/Ca ratios.The relatively high Sr contents of biotic calcite result from rapid precipitation rates associated with shell accretion in marine organisms. Calcites precipitated from seawater in laboratory experiments have D(sr) values that are similar to those of biotic marine calcite, suggesting that both precipitate at approximately the same rate. Our estimates of surface area-normalized precipitation rates in planktonic and benthonic foraminifera are comparable to those of seeded, pH-stat experiments. We conclude that the D(sr) values for biotic and experimental marine calcite are kinetically controlled, whereas the lower precipitation rates of abiotic marine calcite yield D(sr) values that approximate equilibrium conditions.Experimentally derived equations describing the relation between D(sr) and calcite precipitation rate indicate that the offset in Sr content between biotic and abiotic calcite is the result of abiotic precipitation rates that are two to five orders of magnitude lower than those of biotic precipitates. However, observations of naturally occurring marine cements suggest that the five-order-of-magnitude offset best represents natural system processes.