Sr isotopes and pore fluid chemistry in carbonate sediment of the Ontong Java Plateau: Calcite recrystallization rates and evidence for a rapid rise in seawater Mg over the last 10 million years
Sr isotopes and pore fluid chemistry in carbonate sediment of the Ontong Java Plateau: Calcite recrystallization rates and evidence for a rapid rise in seawater Mg over the last 10 million years
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DOI：
10.1016/j.gca.2006.06.009
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发表时间：
2006-08-01
影响因子：
5
通讯作者：
DePaolo, Donald J.
中科院分区：
文献类型：
--
作者：
Fantle, Matthew S.;DePaolo, Donald J.
Sr-87/Sr-86比值和Sr浓度在沉积物和孔隙流体中被用来评估方解石重结晶率在ODP网站807 A的Ontong爪哇高原，800米厚的部分碳酸盐软泥和白垩。利用数值模型对某储层段的孔隙流体化学和Sr同位素进行了评价。推断出的方解石重结晶速率在剖面顶部附近为每百万年2%（%/Myr），在剖面较老的部分系统性地降低，使得速率接近0.1/年龄（以年为单位）。推导出的重结晶速率具有重要的意义的解释中的Ca和Mg的浓度分布的孔隙流体。方解石重结晶对孔隙流体化学的影响用反应长度L来描述，L因元素而异，并取决于孔隙流体和固体中的浓度。当L小于沉积剖面的厚度时，孔隙流体浓度由与固相的平衡或稳态交换控制，除了在沉积物-水界面的距离L内。当L相对于沉积物厚度较大时，孔隙流体浓度主要受边界条件和扩散控制。Ca、Sr和Mg的L值分别约为15、150和1500米。L-Sr来自同位素数据和建模，并允许我们推断L-Ca和L-Mg的值。LCa的小值表明，孔隙流体Ca浓度，这逐渐增加的部分，必须是平衡值，保持溶解沉淀交换与方解石，并不反映钙源内或以下的沉积柱。孔隙流体Ca测量和测量的碱度使我们能够计算孔隙流体中的原位pH值，其在400-800 mbsf下从沉积物-水界面附近的7.6降低到7.1 +/- 0.1。虽然计算的pH值与ODP Leg 130期间测量的一些值一致，但大多数测量结果都是人为因素。L-Mg的大值表明，807 A处的孔隙流体Mg浓度不受方解石流体平衡的控制，而是由沉积过程中海水中Mg浓度的变化决定，并由孔隙流体中的水扩散进行修改。我们使用的孔隙流体镁浓度分布在现场807 A检索全球记录的海水镁在过去的35万年，这表明，海水镁迅速增加，在过去的10万年，而不是逐渐在过去的60万年。这一观察表明，新生代海水镁的上升是由大陆风化输入，而不是通过与洋壳交换控制。硅酸盐和碳酸盐的反应速率和年龄之间的关系是惊人的相似，这表明反应亲和力不是硅酸盐溶解速率的主要决定因素。(c)2006年爱思唯尔公司All rights reserved.
The Sr-87/Sr-86 ratios and Sr concentrations in sediment and pore fluids are used to evaluate the rates of calcite recrystallization at ODP Site 807A on the Ontong Java Plateau, an 800-meter thick section of carbonate ooze and chalk. A numerical model is used to evaluate the pore fluid chemistry and Sr isotopes in an accumulating section. The deduced calcite recrystallization rate is 2% per million years (%/Myr) near the top of the section and decreases systematically in older parts of the section such that the rate is close to 0.1/age (in years). The deduced recrystallization rates have important implications for the interpretation of Ca and Mg concentration profiles in the pore fluids. The effect of calcite recrystallization on pore fluid chemistry is described by the reaction length, L, which varies by element, and depends on the concentration in pore fluid and solid. When L is small compared to the thickness of the sedimentary section, the pore fluid concentration is controlled by equilibrium or steady-state exchange with the solid phase, except within a distance L of the sediment-water interface. When L is large relative to the thickness of sediment, the pore fluid concentration is mostly controlled by the boundary conditions and diffusion. The values of L for Ca, Sr, and Mg are of order 15, 150, and 1500 meters, respectively. L-Sr is derived from isotopic data and modeling, and allows us to infer the values of L-Ca and L-Mg. The small value for LCa indicates that pore fluid Ca concentrations, which gradually increase down section, must be equilibrium values that are maintained by solution-precipitation exchange with calcite and do not reflect Ca sources within or below the sediment column. The pore fluid Ca measurements and measured alkalinity allow us to calculate the in situ pH in the pore fluids, which decreases from 7.6 near the sediment-water interface to 7.1 +/- 0.1 at 400-800 mbsf. While the calculated pH values are in agreement with some of the values measured during ODP Leg 130, most of the measurements are artifacts. The large value for L-Mg indicates that the pore fluid Mg concentrations at 807A are not controlled by calcite-fluid equilibrium but instead are determined by the changing Mg concentration of seawater during deposition, modified by aqueous diffusion in the pore fluids. We use the pore fluid Mg concentration profile at Site 807A to retrieve a global record for seawater Mg over the past 35 Myr, which shows that seawater Mg has increased rapidly over the past 10 Myr, rather than gradually over the past 60 Myr. This observation suggests that the Cenozoic rise in seawater Mg is controlled by continental weathering inputs rather than by exchange with oceanic crust. The relationship determined between reaction rate and age in silicates and carbonates is strikingly similar, which suggests that reaction affinity is not the primary determinant of silicate dissolution rates in nature. (c) 2006 Elsevier Inc. All rights reserved.