Chemical Evolution Of Seawater Research Articles

Lithium elemental and isotope systematics of modern and cultured brachiopods: Implications for seawater evolution

Lithium has proven a powerful tracer of weathering processes and chemical evolution of seawater. Skeletal components of marine calcifying organisms, and in particular brachiopods, present promising archives of Li signatures. However, Li incorporation mechanisms and potential influence from biological processes or environmental conditions require a careful assessment. In order to constrain Li systematics in brachiopod shells, we present Li concentrations and isotope compositions for 11 calcitic brachiopod species collected from six different geographic regions, paralleled with data from culturing experiments where brachiopods were grown under varying environmental conditions and seawater chemistry (pH–pCO2, temperature, Mg/Ca ratio). The recent brachiopod specimens collected across different temperate and polar environments showed broadly consistent δ7Li values ranging from 25.2 to 28.1‰ (with mean δ7Li of 26.9 ± 1.5‰), irrespective of taxonomic rank, indicating that incorporation of Li isotopes into brachiopod shells is not strongly affected by vital effects related to differences among species. This results in Δ7Licalcite–seawater values (per mil difference in 7Li/6Li between brachiopod calcite shell and seawater) from −2.9‰ to −5.8‰ (with mean Δ7Licalcite–seawater value of −3.6‰), which is larger than the Δ7Licalcite–seawater values calculated based on data from planktonic foraminifera (~0‰ to ~−4‰). This range of values is further supported by results from brachiopods cultured experimentally. Under controlled culturing conditions simulating the natural marine environment, the Δ7Licalcite–seawater for Magellania venosa was −2.5‰ and not affected by an increase in temperature from 10 to 16 °C. In contrast, a decrease in Mg/Ca (or Li/Ca) ratio of seawater by addition of CaCl2 as well as elevated pCO2, and hence low-pH conditions, resulted in an increased Δ7Licalcite-seawater up to −4.6‰. Collectively, our results indicate that brachiopods represent valuable archives and provide an envelope for robust Li-based reconstruction of seawater evolution over the Phanerozoic.

Chemical evolution of seawater during the Phanerozoic: Implications from the record of marine evaporites

The chemical evolution of seawater during the Phanerozoic is still a matter of debate. We have assembled and critically analyzed the available data for the composition of fluid inclusions in marine halite and for the mineralogy of marine evaporites. The composition of fluid inclusions in primary marine halite reveals two major long-term cycles in the chemistry of seawater during the past 600 myr. The concentration of Mg2+, Ca2+, and SO42− has varied quite dramatically. The Mg2+ concentration in seawater during most of the early Paleozoic and Jurassic to Cretaceous was as low as 30 to 40 mmol/kg H2O; it reached maximum values ≥50 mmol/kg H2O during the Late Neoproterozoic and Permian. The Ca2+ concentration in seawater during the Phanerozoic has reached maximum values two to three times greater than the concentration in seawater today (10.6 mmol/kg H2O), whereas SO42− concentrations may have been as low as 5 to 10 mmol/kg H2O (a third to a fifth of the modern value) during the Jurassic and Early Paleozoic. The Mg2+/Ca2+ ratio in seawater ranged from 1 to 1.5 during the early to middle Paleozoic and Jurassic-Cretaceous to a near-modern value of 5.2 during the Late Neoproterozoic and Permian. This change in seawater Mg2+/Ca2+ ratio is consistent with the notion of alternating “calcite-aragonite seas” recorded in oölites and marine carbonate cements.Several models have been proposed to explain the chemical evolution of seawater. These have invoked significant changes in one or more of the major geochemical processes that control the composition of seawater. The pattern and magnitude of the variations in the composition of seawater proposed in this study are similar to those proposed elsewhere that suggest that seawater fluxes through midocean ridges have played a major role in the evolution of seawater during the past 600 myr. Two Phanerozoic supercycles of the Earth’s exogenic processes were recognized in the literature that are caused by mantle convection and plate activity. The composition of seawater has apparently undergone dramatic secular changes in phase with these supercycles and as a consequence of biological evolution. Analyses of fluid inclusions containing unevaporated seawater and a better understanding of the processes that affect the composition of seawater are needed to refine our understanding of the history of Phanerozoic seawater.

The 187Os/188Os Record of Himalayan Palaeorivers: Himalayan Tectonics and Ocean Chemistry

Significant debate exists on the role of the uplift and unroofing of the Himalayan mountain range on the chemical evolution of seawater, atmospheric C02 consumption and climate change. Weathering of silicate minerals as the Himalayas were uplifted would consume significant quantities of atmospheric CQ, possibly sufficient to cool the earth's atmosphere (Raymo and Ruddiman, 1992). Variations in the S7Sr/S6Sr record of the oceans has thus been used as proxy for the extent of silicate weathering of the high Himalayas during uplift, due to the large volumes of sediment carried by Himalayan river systems (e.g. Edmond, 1992). However recently, Quade et al., (1997) has suggested that the source of Sr measured in Himalayan rivers are from carbonates (which do not consume CO2 during weathering) from the foothills and lesser Himalaya. Therefore, the marine Sr record may not be a proxy for greater Himalayan unroofing, silicate weathering, COs consumption or change in global climate. Seawater lSVOs/188Os ratios have undergone a rapid rise over the past 15 Ma (e.g. Ravizza, 1993). Currently debate exists as to whether this increase can be attributed in part to the increase in continental erosion due to Himalayan uplift, in analogy with the marine Sr isotopic record. The evolution of the osmium isotopic ratio of seawater provides another means of deciphering the relative contributions of continental weathering to the chemical evolution of seawater, aTsr and 187Os are radiogenic daughter products of aTRb and lS7Re, respectively. Changes in the isotopic ratios of Os and Sr are therefore dependent on time and the amount of the parent isotope present, however Re and Os are hosted by different minerals than Rb and Sr. Thus combining the Os and Sr isotopic records, which are distinct geochemical and mineralogic systems, we can constrain the geologic process(es) that affect ocean chemistry and atmospheric C Q consumption. Quade et al, (1997) focused on carbonates from palaeosoils of rivers draining the Himalayas and western Nepal. The palaeosoils form in floodplains

Neoproterozoic variations in the C-isotopic composition of seawater: stratigraphic and biogeochemical implications

The recent proliferation of stratigraphic studies of δ 13C variation in carbonates and organic C in later Neoproterozoic and basal Cambrian successions (∼ 850-530 Ma) indicates a strong oscillating trend in the C-isotopic composition of surface seawater. Alone, this trend does not adequately characterize discrete intervals in Neoproterozoic time. However, integrated with the vectorial signals provided by fossils and Sr-isotopic variations, C isotope chemostratigraphy facilitates the interbasinal correlation of later Neoproterozoic successions. Results of these studies are evaluated in terms of four stratigraphic intervals: (1) the Precambrian/Cambrian boundary, (2) the post-Varanger terminal Proterozoic, (3) the late Cryogenian, and (4) the early Cryogenian. Where biostratigraphic or radiometric data constrain the age of Neoproterozoic sedimentary sequences, secular variations in C and Sr isotopes can provide a level of stratigraphic resolution exceeding that that provided by fossils alone. Isotopic data place strong constraints on the chemical evolution of seawater, linking it to major tectonic and paleoclimatic events. They also provide a biogeochemical framework for the understanding of the initial radiation of macroscopic metazoans, which is associated stratigraphically, and perhaps causally, with a global increase in the burial of organic C and a concomitant rise of atmospheric O 2.

Silicate-sea water equilibria in the ocean system

The ocean system can be viewed as a composite of local equilibrium states involving sea water and sedimentary material of variable composition. The chemical potentials of the major chloride components of sea water are functions solely of temperature and pressure in the present ocean. This behavior decreases the maximum number of phases that may coexist at equilibrium in the system. Stability fields of aluminosilicates (including a mixed-layer illite-montmorillonite solid solution) in a nine component model of the system can be depicted on isothermal-isobaric activity diagrams in terms of pH and log α H 4SiO 4 . Because these variables change significantly with depth and geographic position in the ocean, the equilibrium composition of the mixed-layer phase also changes, becoming less illitic with increasing depth. Quantitative consideration of equilibria in the present ocean provides a frame of reference for investigation of the chemical evolution of sea water and the mass transfer involved in silicate-sea water reactions through geologic time.