THE MAJOR-ION COMPOSITION OF CENOZOIC SEAWATER: THE PAST 36 MILLION YEARS FROM FLUID INCLUSIONS IN MARINE HALITE
THE MAJOR-ION COMPOSITION OF CENOZOIC SEAWATER: THE PAST 36 MILLION YEARS FROM FLUID INCLUSIONS IN MARINE HALITE
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DOI：
10.2475/08.2013.01
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发表时间：
2013-10-01
影响因子：
2.9
通讯作者：
Cendon, Dioni I.
中科院分区：
文献类型：
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作者：
Brennan, Sean T.;Lowenstein, Tim K.;Cendon, Dioni I.
利用10个新生代（始新世-中新世）海相岩盐的流体包裹体定量分析了过去36年来海水的主要离子组成（Mg2+、Ca2+、K+、Na+、SO42-和Cl-）。用于确定岩盐的海水来源的标准包括：(1)地层、沉积学和古生物学观察；(2)卤代溴；(3)硫酸盐矿物S-34 δ；(4)碳酸盐和硫酸盐的Sr-87/Sr-86；(5)流体包裹体卤水组成和蒸发路径，它们必须在地理上分开的相同年龄的盆地中重叠，以确认“全球”海水化学信号。新生代海水主离子化学的变化标志着从富Ca2+、Mg2+和SO42-的中生代海水（“CaCl2海”）到富Mg2+和SO42- >Ca2+的新生代“MgSO4海”（高Mg2+和SO42- >Ca2+）的系统的、长期的（>50）转变的结束。计算了始新世-渐新世（36-34 Ma）、塞拉瓦里安-托尔顿期（13.5-11.8 Ma）和墨西尼亚期（6-5 Ma）新生代海水的主要离子组成，假设氯度（565 mmol）、盐度和K+浓度（11 mmol）不变，与现代海水相同。新生代海相岩盐流体包裹体表明，近36年来海水中Mg2+和SO42-浓度增加，Ca2+浓度下降。Mg2+浓度从始新世-渐新世海水（36-34 Ma）的36 mmol / l上升到现代海水的55 mmol / l。始新世末海水的Mg2+/Ca2+比值为2.3,13.5 ~ 11.8 Ma和6 ~ 5 Ma时分别为3.4和4.0，近现代海水的Mg2+/Ca2+比值也有所上升。始新世-渐新世海水（36-34 Ma） SO42- = 14-23 mmol / l, Ca2+ = 11-20 mmol / l。如果假设（Ca2+）（SO42-）产物与现代海水相同（类似于300 mmol(2)），始新世-渐新世海水的Ca2+和SO42-分别为16 mmol和19 mmol。在Serravallian-Tortonian海水（13.5-11.8 Ma）中，如果（Ca2+） （SO42-）产物与现代海水相同，则Ca2+和SO42-的估计值为SO42- = 19-27 mmol, Ca2+ = 8-16 mmol， SO42-与24 mmol相似，Ca2+与13 mmol相似。假设（Ca2+） （SO42-）与现代海水的产物相等，则墨西inian海水的SO42-与21-29 mmol相似，Ca2+与7-15 mmol相似，SO42-与26 mmol相似，Ca2+与12 mmol相似。无论采用何种估算方法，从36 Ma到现在，SO42-的浓度逐渐增加，是新生代的最高值。
Fluid inclusions from ten Cenozoic (Eocene-Miocene) marine halites are used to quantify the major-ion composition (Mg2+, Ca2+, K+, Na+, SO42-, and Cl-) of seawater over the past 36 My. Criteria used to determine a seawater origin of the halites include: (1) stratigraphic, sedimentologic, and paleontologic observations; (2) Br- in halite; (3) delta S-34 of sulfate minerals; (4) Sr-87/Sr-86 of carbonates and sulfates; and (5) fluid inclusion brine compositions and evaporation paths, which must overlap from geographically separated basins of the same age to confirm a "global" seawater chemical signal.Changes in the major-ion chemistry of Cenozoic seawater record the end of a systematic, long term (>150 My) shift from the Ca2+-rich, Mg2+- and SO42--poor seawater of the Mesozoic ("CaCl2 seas") to the "MgSO4 seas" (with higher Mg2+ and SO42- >Ca2+) of the Cenozoic. The major ion composition of Cenozoic seawater is calculated for the Eocene-Oligocene (36-34 Ma), Serravallian-Tortonian (13.5-11.8 Ma) and the Messinian (6-5 Ma), assuming chlorinity (565 mmolal), salinity, and the K+ concentration (11 mmolal) are constant and the same as in modern seawater. Fluid inclusions from Cenozoic marine halites show that the concentrations of Mg2+ and SO42- have increased in seawater over the past 36 My and the concentration of Ca2+ has decreased. Mg2+ concentrations increased from 36 mmolal in Eocene-Oligocene seawater (36-34 Ma) to 55 mmolal in modern seawater. The Mg2+/Ca2+ ratio of seawater has risen from similar to 2.3 at the end of the Eocene, to 3.4 and 4.0, respectively, at 13.5 to 11.8 Ma and 6 to 5 Ma, and to Sin modern seawater.Eocene-Oligocene seawater (36-34 Ma) has estimated ranges of SO42- = 14-23 mmolal and Ca2+ = 11-20 mmolal. If the (Ca2+)(SO42-) product is assumed to be the same as in modern seawater (similar to 300 mmolal(2)), Eocene-Oligocene seawater had Ca2+ similar to 16 mmolal and SO42- similar to 19 mmolal. The same estimates of Ca2+ and SO42- for Serravallian-Tortonian seawater (13.5-11.8 Ma) are SO42- = 19-27 mmolal and Ca2+ = 8-16 mmolal and SO42- similar to 24 mmolal and Ca2+ similar to 13 mmolal if the (Ca2+) (SO42-) product is equal to that in modern seawater. Messinian seawater has an estimated range of SO42- similar to 21-29 mmolal and Ca2+ similar to 7-15 mmolal with SO42- similar to 26 mmolal and Ca2+ similar to 12 mmolal assuming the (Ca2+) (SO42-) product is equal to that in modern seawater. Regardless of the estimation procedure, SO42- shows progressively increasing concentrations from 36 Ma to the present values, which are the highest of the Cenozoic.