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Lau2017a.Pdf Chemical Geology 471 (2017) 13–37 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo The influence of seawater carbonate chemistry, mineralogy, and diagenesis MARK on calcium isotope variations in Lower-Middle Triassic carbonate rocks ⁎ Kimberly V. Laua, , Kate Mahera, Shaun T. Brownb,c, Adam B. Josta,d, Demir Altınere, Donald J. DePaolob,c, Anton Eisenhauerf, Brian M. Kelleya,g, Daniel J. Lehrmannh, Adina Paytani, Meiyi Yuj, Juan Carlos Silva-Tamayoa,k, Jonathan L. Paynea a Department of Geological Sciences, Stanford University, 450 Serra Mall Bldg. 320, Stanford, CA 94305, USA b Department of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94709, USA c Energy Geosciences Division, E.O. Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA d Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA e Department of Geological Engineering, Middle East Technical University, Üniversiteler Mah. Dumlupınar Blv. No: 1, 06800, 06531 Ankara, Turkey f GEOMAR, Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany g ExxonMobil Upstream Research Company, 22777 Springwoods Village Parkway, Spring, TX 77389, USA h Geosciences Department, Trinity University, Marrs McLean Hall, One Trinity Place, San Antonio, TX 78212, USA i Institute of Marine Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA j College of Resource and Environment Engineering, Guizhou University, Caijiaguan, Guiyang 550003, Guizhou, China k TESTLAB Geo-Ambiental, Kra 45D #60-16, Medellin, Colombia ARTICLE INFO ABSTRACT Editor: Michael E. Böttcher The geological calcium cycle is linked to the geological carbon cycle through the weathering and burial of 44 40 δ44/40 Keywords: carbonate rocks. As a result, calcium (Ca) isotope ratios ( Ca/ Ca, expressed as Ca) can help to constrain 44/40 Calcium isotopes ancient carbon cycle dynamics if Ca cycle behavior can be reconstructed. However, the δ Ca of carbonate Carbonates rocks is influenced not only by the δ44/40Ca of seawater but also by diagenetic processes and fractionation Early diagenesis associated with carbonate precipitation. In this study, we investigate the dominant controls on carbonate δ44/ Recrystallization 40Ca in Upper Permian to Middle Triassic limestones (ca. 253 to 244 Ma) from south China and Turkey. This time Calcium cycling interval is ideal for assessing controls on Ca isotope ratios in carbonate rocks because fluctuations in seawater δ44/40Ca may be expected based on several large carbon isotope (δ13C) excursions ranging from −2to+8‰. Parallel negative δ13C and δ44/40Ca excursions were previously identified across the end-Permian extinction horizon. Here, we find a second negative excursion in δ44/40Ca of ~0.2‰ within Lower Triassic strata in both south China and Turkey; however, this excursion is not synchronous between regions and thus cannot be in- terpreted to reflect secular change in the δ44/40Ca of global seawater. Additionally, δ44/40Ca values from Turkey are consistently 0.3‰ lower than contemporaneous samples from south China, providing further support for local or regional influences. By measuring δ44/40Ca and Sr concentrations ([Sr]) in two stratigraphic sections located at opposite margins of the Paleo-Tethys Ocean, we can determine whether the data represent global conditions (e.g., secular variations in the δ44/40Ca of seawater) versus local controls (e.g., original mineralogy or diagenetic alteration). The [Sr] and δ44/40Ca data from this study are best described statistically by a log-linear correlation that also exists in many previously published datasets of various geological ages. Using a model of early marine diagenetic water-rock interaction, we illustrate that this general correlation can be explained by the chemical evolution of bulk carbonate sediment samples with different initial mineralogical compositions that subsequently underwent recrystallization. Although early diagenetic resetting and carbonate mineralogy strongly influence the carbonate δ44/40Ca values, the relationship between [Sr] and δ44/40Ca holds potential for reconstructing first-order secular changes in seawater δ44/40Ca composition. ⁎ Corresponding author at: Department of Earth Sciences, University of California, Riverside, 900 University Avenue, Riverside, CA 92521, USA. E-mail address: [email protected] (K.V. Lau). http://dx.doi.org/10.1016/j.chemgeo.2017.09.006 Received 23 May 2017; Received in revised form 18 August 2017; Accepted 6 September 2017 Available online 08 September 2017 0009-2541/ © 2017 Elsevier B.V. All rights reserved. K.V. Lau et al. Chemical Geology 471 (2017) 13–37 1. Introduction return to pre-extinction levels until the Middle Triassic, suggesting that adverse conditions extended beyond the initial extinction event (e.g., Records of calcium isotopes (δ44/40Ca), measured in marine carbo- Brayard et al., 2006, 2009; Chen and Benton, 2012; Foster and nate rocks, fossils, and authigenic minerals, provide important con- Twitchett, 2014; Hallam, 1991; Orchard, 2007; Schaal et al., 2016; straints on global calcium (Ca) and carbon (C) cycle dynamics over the Stanley, 2009). Because δ44/40Ca data can be collected from the same past billion years (e.g., Blättler et al., 2011; Brazier et al., 2015; De La samples as those used to determine variations in δ13C, this time interval Rocha and DePaolo, 2000; Du Vivier et al., 2015; Fantle and DePaolo, serves as an important test case for the utility of δ44/40Ca data in car- 2005; Farkaš et al., 2007, 2016; Gothmann et al., 2016; Griffith et al., bonate rocks for reconstructing seawater δ44/40Ca, as well as for elu- 2008a, 2011; Holmden, 2009; Holmden et al., 2012a; Husson et al., cidating links between the Early-Middle Triassic Ca and C cycles. 2015; Jost et al., 2014, 2017; Kasemann et al., 2014; Payne et al., 2010; At the Permian/Triassic boundary, the globally observed negative 44/ 13 13 Sawaki et al., 2014; Silva-Tamayo et al., 2010a, 2010b). Seawater δ δ C excursion is commonly attributed to a release of C-depleted CO2 40Ca may reflect coupled C and Ca cycling because its value is con- during Siberian Traps volcanism and volatilization of C-rich sediments trolled by the balance between the major Ca input, continental (e.g., Cui et al., 2013; Svensen et al., 2009). Because rapidly elevated weathering, and the major Ca output, CaCO3 burial (De La Rocha and atmospheric pCO2 would increase Ca weathering as well as decrease the 44/40 DePaolo, 2000; DePaolo, 2004; Fantle and DePaolo, 2005). During burial rate of CaCO3, a negative excursion in δ Ca that parallels this 40 13 CaCO3 precipitation, Ca is fractionated such that Ca is preferentially negative δ C excursion was interpreted as evidence for a major episode incorporated into carbonate minerals, enriching seawater in 44Ca re- of ocean acidification (Hinojosa et al., 2012; Payne et al., 2010). A lative to the riverine input (e.g., DePaolo, 2004). Assuming that riverine number of additional lines of evidence support an end-Permian acid- δ44/40Ca does not vary significantly across time, the δ44/40Ca record of ification event, including high 187Re/188Os ratios in shales (Georgiev carbonate sediments could reflect changes in carbonate mineralogy or et al., 2011), the paleophysiology of animals that were most severely the sedimentation flux. If the carbonate burial flux increases due to impacted (Clapham and Payne, 2011; Kiessling and Simpson, 2011; higher alkalinity (with no contemporaneous changes in hydrothermal Knoll et al., 2007), sedimentological observations such as dissolution input or groundwater flux), seawater δ44/40Ca also increases. On the surfaces and subsequent abiotic carbonate precipitation (Baud et al., other hand, if carbonate saturation state (Ω) decreases temporarily in 2007; Payne et al., 2007; Pruss et al., 2006; Weidlich and Bernecker, an acidified ocean, reduced carbonate sedimentation results in a tran- 2011; Woods et al., 1999; Woods, 2014), and a negative boron isotope sient decrease in seawater δ44/40Ca (Payne et al., 2010). In this study, excursion that suggests a short-lived acidification event (Clarkson et al., we examine the potential for Ca isotopes, as recorded in carbonate 2015). rocks, to provide insight into the coupled C and Ca cycles by focusing In contrast to the boundary δ13C excursion, the causes of Early on Lower to Middle Triassic carbonate strata from stratigraphic sections Triassic δ13C instability are widely debated. The large negative and located in south China and Turkey. positive δ13C excursions that characterize the remainder of the Early Carbonate δ44/40Ca records can provide constraints on marine C and Triassic have been interpreted to represent several pulses of volcanic C Ca cycling if variations in these data reflect variations in seawater δ44/ release (Payne and Kump, 2007). However, radiometric dates of Si- 40Ca. Calcium isotopes are readily analyzed in bulk carbonate rock, an berian Traps rocks indicate that magmatism greatly diminished within approach that is useful for time intervals prior to the Cenozoic that are 0.5 Ma of the main extinction pulse (Burgess and Bowring, 2015), not represented in the deep-sea sediment record. However, various suggesting that volcanic pulses are unlikely to have continued for 5 Ma. factors can affect the extent to which the δ44/40Ca composition of bulk Nonetheless,
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