Tim Sweere

Last Name

Sweere

First Name

Tim

ORCID

0000-0001-8420-7209

Organisational unit

03956 - Vance, Derek / Vance, Derek

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Publications 1 - 5 of 5

Environmental controls on very high δ238U values in reducing sediments: Implications for Neoproterozoic seawater records
Clarkson, Matthew O.; Sweere, Tim; Chiu, Chun Fung; et al. (2023)
Earth-Science Reviews
Uranium isotopes (δ238U) are a widely applied tool for tracing global changes in oceanic anoxia. Interpretation of seawater values and trends, often reconstructed from carbonates, requires knowledge of the U isotope fractionation that occurs during U reduction, typically favouring the heavier 238U isotope relative to 235U. Yet the environmental controls on the expression of isotope enrichment during reduction (Δ238Uanox) are poorly understood, leading to large uncertainties in interpretation of seawater records. This is particularly limiting for the Neoproterozoic, where exceptionally low inferred seawater δ238U requires very high Δ238Uanox, which are rarely seen in modern sediments. Here we present a compilation of authigenic δ238U from modern and recent (Mediterranean sapropel) reducing settings to better constrain the first order controls on the expression of large U isotope enrichments. Accompanying geochemical data help identify the dominant mechanisms responsible for high Δ238Uanox, suggesting they are an expression of limited sedimentary U reduction in weakly euxinic settings or temporally dynamic reducing environments. Such environments are characterised by lower to intermediate organic carbon and uranium accumulation rates (OCAR, UAR) where U reduction appears dominated by non-diffusion-limited processes at the sediment-water interface, on sinking organic matter or within the water column itself. Conversely, under strongly euxinic conditions with higher OCAR and UAR, U reduction occurs mainly under a diffusion-limited regime in the sediment. These findings suggest that the very low seawater δ238U of the Neoproterozoic may be a result of progressive ocean oxygenation and temporally dynamic expansions of anoxia, or the development of weakly euxinic conditions, rather than more widespread or ‘intense’ anoxia as previously inferred. Such a revised interpretation is more consistent with other geochemical and paleontological records from this time and is critical for understanding the relationship of anoxia to the rise of complex life.
Nickel and zinc micronutrient availability in Phanerozoic oceans
Sweere, Tim; Dickson, Alexander J.; Vance, Derek (2023)
Geobiology
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 longterm 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 similar to 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 similar to 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 predates 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.
Co-variation systematics of uranium and molybdenum isotopes reveal pathways for descent into euxinia in Mediterranean sapropels
Chiu, Chun Fung; Sweere, Tim; Clarkson, Matthew O.; et al. (2022)
Earth and Planetary Science Letters
The elemental concentrations and isotope compositions of molybdenum (Mo) and uranium (U) are commonly used for the reconstruction of past global and local redox conditions, and recent studies using both elements have revealed the potential of their paired application. However, such studies have generally focused either on modern marine sediments or on relatively low-resolution reconstructions of deep-time paleo-redox conditions. Here, we present high-resolution profiles (every 0.2-0.3 kyr) of Mo and U elemental and isotope compositions for anoxic organic-rich sediments of Eastern Mediterranean sapropels S5 and S7. The new Mo-U data reveal the processes leading to descent into basinal euxinia in more precise and systematic detail than lower resolution datasets focused on either Mo or U only. During the intensification from anoxic non-sulfidic to persistently euxinic conditions, δ98Mo and δ238U covariation systematics exhibit two stages. We identify the Mo-U isotope signature of the early transition from anoxic non-sulfidic to mildly euxinic conditions in the first stage, characterized by a rise in U isotope ratios (from −0.3‰ to +0.2‰ ±0.05‰) controlled by the depth within the sediment of the uranium reduction-accumulation front. As the water column turns persistently euxinic, δ98Mo values approach the seawater value for both sapropels, but δ238U evolves to different final values in the two sapropels. We interpret these differences as reflecting different redox potentials and/or different degrees of restriction of these two sapropel events, and the more gradual response of U sequestration to redox variation in comparison with threshold behavior of Mo. The findings presented here show temporal patterns in δ98Mo and δ238U on relatively short timescales that suggest the combined use of these proxies at high resolution allows detailed reconstruction of local redox and hydrographic conditions.
Trace metal evolution of the Late Cretaceous Ocean
Sun, Mingzhao; Archer, Corey; Scholz, Florian; et al. (2025)
Chemical Geology
The Cenomanian-Turonian boundary (Late Cretaceous) witnessed the last spectacular manifestation of Mesozoic Anoxic Events (OAE 2, ∼94 Ma), marked by a prominent carbon isotope excursion (CIE) and burial of organic-matter-rich sediments under high atmospheric CO2 concentrations. But the Late Cretaceous generally was a time of profound environmental change. OAE 2 was preceded by other CIEs, including the Mid-Cenomanian Event (MCE), and was punctuated by a short re‑oxygenation and cooling event (the Plenus Cold Event, PCE). Extensive previous studies, including many trace metal studies, have focused on OAE 2, but there is still debate concerning the degree of drawdown of oceanic trace metal reservoirs during OAE 2, whether this drawdown is global or local, its causes and consequences for ocean ecology. Here, we present records of eight trace metals, over about 5 Myr of the Late Cretaceous, from the Tarfaya Basin in the proto-North Atlantic. The long records from a core preserving a continuous sedimentary succession allow us to set changes occurring across OAE 2 in the broader context of Late Cretaceous, including the lead up to OAE 2. Moreover, the multiple trace metal dataset allows us to broadly investigate the oceanographic setting in the context of recent studies of multiple trace metals in modern organic-rich sediments aimed at refining the proxies. Trace metal enrichments in these organic-rich sediments are discussed on three different timescales. Firstly, comparison of these Late Cretaceous sediments with modern organic-rich sediments are consistent with deposition in an open ocean upwelling margin in the Late Cretaceous, very like the modern Peru or Namibian Margin, although the deep proto-North Atlantic was probably partially restricted. Secondly, in common with previous studies, metal/TOC ratios often show sharp drops in the early part of OAE 2. Thirdly, however, this sharp drop occurs within a framework of pseudo-cyclical variations in metal/TOC, with a period of about 143 ±19 kyr (1 SD), that is a feature of these long records well before OAE 2, including across the MCE. Different metals respond differently to the perturbation in the early part of OAE 2 itself. Simple mass balance considerations suggest that trace metal drawdown with organic carbon must be at least partially compensated by changes in the rate of chemical weathering on the continents, as previously inferred from Li and Ca isotopes. Moreover, changes in the patterns of variation between different metals, as well as covariation of metal/TOC ratios and Os isotopes, hint at changes in the pattern of chemical weathering, most prominently in the contribution of mafic rocks to the chemical weathering flux.
Ni isotope cycling in sediments of highly productive upwelling systems
Sweere, Tim; Archer, Corey; Fleischmann, Sarah; et al. (2025)
Earth and Planetary Science Letters
Nickel is a bio-essential micronutrient in the ocean and the element holds significant potential for the reconstruction of paleoenvironmental conditions. Nickel isotopes provide information on processes controlling internal oceanic cycling and the oceanic mass balance. These insights provide the basis for inferences from the sedimentary record. Recent studies have shown that diagenetic Ni cycling can cause isotope fractionation and may affect the global oceanic dissolved pool, but direct data to constrain such processes in detail remain relatively scarce. Here, we present a large Ni isotope dataset for sediments and pore waters at eight different locations along the Namibian and Peruvian margins, some of the most productive and oxygen-depleted regions in the open ocean. These different stations represent a wide range of depositional redox conditions, allowing evaluation of Ni behaviour in different environments and better constraints on sedimentary Ni cycling and the potential of Ni as a paleo-environmental proxy. The sedimentary data presented here reinforce findings from previous studies that have suggested that excess Ni burial in organic-rich, strongly reducing sediments underlying highly productive surface waters is unfractionated from the deep ocean isotope composition (1.33 ± 0.07 ‰), with an average δ60Niexcess of 1.37 ± 0.16 ‰ (1 SD, n = 74) for Ni-enriched samples (fexcess >0.8). This observation suggests that sedimentary records from organic-rich continental margin upwelling settings can serve as an archive for past deep-ocean δ60Ni values. The pore water data provide additional constraints on Ni isotope cycling in different redox conditions and show that the preferential precipitation of isotopically light Ni leaves sulphidic pore waters enriched in isotopically heavy Ni, with an average δ60Ni value of 2.04 ± 0.41. Pore-water Ni concentrations are higher than those in the overlying bottom water, implying a diffusive benthic source in these environments, with an average local flux of -28 μmol m-2 yr-1. Where outputs from the oceanic dissolved pool are defined in terms of the characteristics of deeper sediments, this benthic flux should be viewed as recycled, not as a new net input in the oceanic mass balance. With other recent studies, our data highlight benthic Ni fluxes as a feature of highly productive environments despite sulphidic pore waters. However, the observed benthic fluxes in these studies are relatively small compared to sedimentary Ni accumulation rates, suggesting Ni burial is relatively efficient, with 92 ± 7 % (1SD, n = 11) of deposited Ni being retained in the sediment.

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Tim Sweere

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