Oxygen isotope composition of the Phanerozoic ocean and a possible solution to the dolomite problem

Abstract

The O/O of calcite fossils increased by ∼8‰ between the Cambrian and present. It has long been controversial whether this change reflects evolution in the δO of seawater, or a decrease in ocean temperatures, or greater extents of diagenesis of older strata. Here, we present measurements of the oxygen and ‟clumped" isotope compositions of Phanerozoic dolomites and compare these data with published oxygen isotope studies of carbonate rocks. We show that the δO values of dolomites and calcite fossils of similar age overlap one another, suggesting they are controlled by similar processes. Clumped isotope measurements of Cambrian to Pleistocene dolomites imply crystallization temperatures of 15-158 °C and parent waters having δO values from -2 to +12‰. These data are consistent with dolomitization through sediment/rock reaction with seawater and diagenetically modified seawater, over timescales of 100 My, and suggest that, like dolomite, temporal variations of the calcite fossil δO record are largely driven by diagenetic alteration. We find no evidence that Phanerozoic seawater was significantly lower in δO than preglacial Cenozoic seawater. Thus, the fluxes of oxygen-isotope exchange associated with weathering and hydrothermal alteration reactions have remained stable throughout the Phanerozoic, despite major tectonic, climatic and biologic perturbations. This stability implies that a long-term feedback exists between the global rates of seafloor spreading and weathering. We note that massive dolomites have crystallized in pre-Cenozoic units at temperatures >40 °C. Since Cenozoic platforms generally have not reached such conditions, their thermal immaturity could explain their paucity of dolomites.

Chemical Composition of Carbonate Hardground Cements as Reconstructive Tools for Phanerozoic Pore Fluids

“…This is consistent with earlier studies of carbonate cements (Carpenter et al, ). The trend of lower δ 18 O in carbonate minerals going back in time through the Phanerozoic is controversial and has been interpreted as either reflecting a change in the δ 18 O of the ocean (such that early Phanerozoic oceans had a δ 18 O that was −4‰ to −6‰ vs. 0‰ today) or to reflect diagenesis, such that the δ 18 O of the carbonate minerals no longer has its initial δ 18 O (Ryb & Eiler, ; Veizer et al, ). Changes to the δ 18 O of carbonate minerals are largely a function of burial depth after deposition and degree to which the carbonate minerals have seen meteoric water, both fluids with a lower δ 18 O than seawater.…”

Effects of different constants and standards on the reproducibility of carbonate clumped isotope (Δ47) measurements: Insights from a long‐term dataset

“…Compared with the conventional carbonate–water isotopic thermometer, the Δ 47 geothermometer can provide quantitative temperature data without a priori knowledge of paleowater δ 18 O values. It can be used to reconstruct the paleotemperature at which a carbonate was precipitated provided that the sample has not been influenced by kinetic isotope effects and has not been significantly altered through diagenesis or solid‐state reordering . Thus, this proxy has potential as a paleotemperature tool for investigation of geological events such as the Snowball Earth deglaciation and Eocene warming .…”

“…Thus, this proxy has potential as a paleotemperature tool for investigation of geological events such as the Snowball Earth deglaciation and Eocene warming . Thus far, it has been used to address many long‐standing questions in Earth sciences where paleowater δ 18 O values are uncertain …”

“…It can be used to reconstruct the paleotemperature at which a carbonate was precipitated provided that the sample has not been influenced by kinetic isotope effects [4][5][6][7] and has not been significantly altered through diagenesis or solidstate reordering. [8][9][10][11] Thus, this proxy has potential as a paleotemperature tool for investigation of geological events such as the Snowball Earth deglaciation 12 and Eocene warming. 13,14 Thus far, it has been used to address many long-standing questions in Earth sciences where paleowater δ 18 O values are uncertain.…”

“…13,14 Thus far, it has been used to address many long-standing questions in Earth sciences where paleowater δ 18 O values are uncertain. 4,5,8,11,[15][16][17][18][19] The Δ 47 thermometer has the potential to provide unique temperature constraints on geological samples, especially in settings where such information may be unobtainable with other geochemical tools. Since the initial proposal of this proxy, a number of different Δ 47 -T calibrations have been published, leading to reconstructed temperature discrepancies of up to 10-15 C based on the same Δ 47 value.…”