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2. Zn and Cd isotopes as proxies for geochemical change in Mesozoic oceans

Author

Sweere, Tim, Dickson, Alexander, Henderson, Gideon, Porcelli, Donald, and Jenkyns, Hugh

Subjects
551.46
Abstract

Mesozoic Oceanic Anoxic Events (OAEs) are characterised by the widespread burial of organic-rich sediments and the spread of low-oxygen marine environments. These intervals represent some of most extreme perturbations to global climate, environment, and ocean chemistry in Earth's history. Understanding the complex interplay of processes that caused and sustained these unusual environmental conditions requires proxies that allow quantification of global changes in ocean chemistry. This study explores the use of Zn- and Cd-isotopes as new palaeo-ocean chemistry proxies by using the Late Cretaceous Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2, ̃ 94 Ma) as a case study. Previous studies on the cycling of these elements in the modern ocean have suggested Cd and Zn may preserve valuable information on past redox and/or nutrient conditions if such signals are preserved in the sedimentary record. The data presented in this thesis suggest a strong control of local redox conditions on the Cd- and Zn-isotope composition of ancient organic-rich marine sediments. When these local effects are accounted for, δ66Zn and δ114Cd stratigraphic profiles offer insight into past change in redox environments at a global scale. Specifically, heavy δ66Zn and δ114Cd values observed during the main phase of the OAE 2 interval can be associated with an increase in organic-rich sediment burial globally, whereas lower values during the Plenus Cold Event interval can be linked to global oxygenation. The data presented in this study highlight the potential of δ114Cd and δ66Zn as new proxies for global changes in ocean redox chemistry. With improved understanding of the cycling of these elements in the modern ocean, these proxies may ultimately be used to quantify the global extent of ocean anoxia for past oceans.

Published
2018

13. Nickel and zinc micronutrient availability in Phanerozoic oceans.

Author

Sweere, Tim C., Dickson, Alexander J., and Vance, Derek

Subjects
*OCEAN, *SURFACE of the earth, *CENOZOIC Era, *NICKEL, *MESOZOIC Era, *ZINC
Abstract

Nickel and zinc are both bio‐essential micronutrients with a nutrient‐like distribution in the modern ocean, but show key differences in their biological functions and geochemical behavior. Eukaryotic phytoplankton, and especially diatoms, have high Zn quotas, whereas cyanobacteria generally require relatively more Ni. Secular changes in the relative availability of these micronutrients may, therefore, have affected the evolution and diversification of phytoplankton. In this study, we use a large compilation of Ni and Zn concentration data for Phanerozoic sediments to evaluate long‐term changes in Ni and Zn availability and possible links to phytoplankton evolution. Modern data suggest that organic‐rich sediments capture the dissolved deep ocean Ni/Zn ratio, regardless of local depositional conditions. We use this observation to constrain Ni/Zn ratios for past oceans, based on data from the sedimentary record. This record highlights long‐term changes in the relative availability of these micronutrients that can be linked to the (bio)geochemical conditions on the Earth's surface. Early Palaeozoic oceans were likely relatively Ni rich, with sedimentary Ni/Zn ratios for this interval mostly being around ~1 or higher. A comparison with Phanerozoic strontium‐, carbon‐, and sulfur‐isotopic records suggests that the late Palaeozoic decrease in sulfidic conditions and increase in hydrothermal inputs and organic‐carbon burial rates caused a shift towards more Zn‐rich conditions. Mesozoic and Cenozoic sediments show relatively Zn‐rich oceans for these time intervals, with sedimentary Ni/Zn ratios mostly being around ~1 or lower. These observations imply that the diversification of the dominant groups of modern eukaryotic phytoplankton occurred in relatively Zn‐rich oceans and that these organisms still carry this signature in their stoichiometries. However, the Phanerozoic transition to a more Zn‐rich ocean pre‐dates the origin and diversification of modern eukaryotes and, therefore, this transition was likely not the main direct cause for eukaryotic diversification in the Mesozoic and Cenozoic Eras. [ABSTRACT FROM AUTHOR]

Published
2023
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16. New Constraints on Global Geochemical Cycling During Oceanic Anoxic Event 2 (Late Cretaceous) From a 6‐Million‐year Long Molybdenum‐Isotope Record

Author

Dickson, Alexander J., Jenkyns, Hugh C., Idiz, Erdem, Sweere, Tim C., Murphy, Melissa J., van den Boorn, Sander H.J.M., Ruhl, Micha, Eldrett, James S., and Porcelli, Donald

Subjects
molybdenum isotopes, cretaceous western interior seaway, eagle ford shale, oceanic anoxic event 2, redox, trace metals
Abstract

Intervals of extreme warmth are predicted to drive a decrease in the oxygen content of the oceans. This prediction has been tested for the acme of short (, Geochemistry, Geophysics, Geosystems, 22 (3), ISSN:1525-2027

Published
2021

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