Michael Kipp

Journal Article Earth and Planetary Science Letters · December 15, 2024 Lower-crustal garnet clinopyroxenite (sometimes termed “arclogite”) fractionation in thick-crustal (>35 km) arc settings presents a compelling model to explain Fe-depletion trends, high oxygen fugacity, and evidence of recent delamination observed in many ... Full text Cite

Magnitude and timescales of Ca isotope variability in human urine: implications for bone mass balance monitoring.

Journal Article Metallomics : integrated biometal science · November 2024 Calcium (Ca) isotopes in blood/urine are emerging biomarkers of bone mineral balance (BMB) in the human body. While multiple studies have investigated Ca isotopes in patients suffering from diseases affecting BMB, comparatively little effort has been devot ... Full text Cite

Reconstructing the depositional environment and diagenetic modification of global phosphate deposits through integration of uranium and strontium isotopes

Journal Article Chemical Geology · September 5, 2024 The geochemistry of phosphate rocks can provide valuable information on their depositional environment and the redox condition of global oceans through time. Here we examine trace metal concentrations and uranium (δ238U, δ234U) and strontium (87Sr/86Sr) is ... Full text Cite

Iron isotope fractionation during partial melting of metapelites and the generation of strongly peraluminous granites

Journal Article Geochimica et Cosmochimica Acta · September 1, 2024 The large variability in Fe isotope ratios of sedimentary rocks (particularly those from the Archean and Proterozoic) contrasts with that of igneous rocks, which display a much more limited range in values. Notably, among igneous rocks, those inferred to f ... Full text Cite

Lower crustal control in the iron isotope systematics of plutonic xenoliths from Adak Island, Central Aleutians, with implications for arc magma geochemistry

Journal Article Geochimica et Cosmochimica Acta · July 15, 2024 We present bulk-rock and mineral Fe isotope data of ultramafic to mafic xenoliths and basaltic to andesitic lavas from Adagdak Volcano (Adak Island, Central Aleutians) to study the effects of early differentiation on the Fe isotopic evolution of island arc ... Full text Cite

Carbonate uranium isotopes record global expansion of marine anoxia during the Toarcian Oceanic Anoxic Event.

Journal Article Proceedings of the National Academy of Sciences of the United States of America · July 2024 The Toarcian Oceanic Anoxic Event (T-OAE; ~183 Mya) was a globally significant carbon-cycle perturbation linked to widespread deposition of organic-rich sediments, massive volcanic CO2 release, marine faunal extinction, sea-level rise, a crisis ... Full text Cite

Exploring uranium isotopes in shark teeth as a paleo-redox proxy

Journal Article Geochimica et Cosmochimica Acta · January 15, 2024 The uranium isotope composition (δ238U) of seawater is a powerful proxy for the extent of marine anoxia. For paleoredox reconstructions, carbonates are the most popular U isotope archive, but they have recently come under increased scrutiny as their δ238U ... Full text Cite

Nitrogen isotopes reveal independent origins of N₂-fixing symbiosis in extant cycad lineages.

Journal Article Nature ecology & evolution · January 2024 Cycads are ancient seed plants (gymnosperms) that emerged by the early Permian. Although they were common understory flora and food for dinosaurs in the Mesozoic, their abundance declined markedly in the Cenozoic. Extant cycads persist in restricted popula ... Full text Cite

Carbon cycling: How much life has ever existed on Earth?

Journal Article Current biology : CB · November 2023 Carbon has cycled through Earth's biosphere for billions of years. New work estimates that life has recycled the equivalent of almost 100 times the Earth's entire carbon reservoir through the biosphere. This highlights life's global impact, providing a ben ... Full text Cite

Carbonate uranium isotopes across Cretaceous OAE 2 in southern Mexico: New constraints on the global spread of marine anoxia and organic carbon burial

Journal Article Palaeogeography, Palaeoclimatology, Palaeoecology · October 15, 2023 Oceanic anoxic events (OAEs) represent discrete intervals of decreased marine oxygen concentrations often associated with volcanism, enhanced organic carbon burial coupled with positive δ13C excursions, and significant biotic turnover. Cretaceous OAE 2 (ca ... Full text Cite

Biological diversification linked to environmental stabilization following the Sturtian Snowball glaciation.

Journal Article Science advances · August 2023 The body fossil and biomarker records hint at an increase in biotic complexity between the two Cryogenian Snowball Earth episodes (ca. 661 million to ≤650 million years ago). Oxygen and nutrient availability can promote biotic complexity, but nutrient (par ... Full text Cite

²³⁸U, ²³⁵U and ²³⁴U in seawater and deep-sea corals: A high-precision reappraisal

Journal Article Geochimica et Cosmochimica Acta · November 1, 2022 Uranium isotope ratios are widely utilized in paleoceanography. The 238U/235U ratio (expressed as δ238U) is leveraged as a proxy for the areal extent of anoxic seafloor, and the 234U/238U ratio (expressed as δ234Usec) tracks riverine and estuarine inputs t ... Full text Cite

A Double-Edged Sword: The Role of Sulfate in Anoxic Marine Phosphorus Cycling Through Earth History

Journal Article Geophysical Research Letters · October 28, 2022 Modern anoxic marine sediments release phosphorus (P) to seawater, driving feedbacks at multiple timescales. On sub-Myr timescales, anoxic P regeneration amplifies ocean deoxygenation; on multi-Myr timescales, it stabilizes atmospheric O2. Some authors hav ... Full text Cite

Unique evidence of fluid alteration in the Kakowa (L6) ordinary chondrite.

Journal Article Scientific reports · April 2022 Meteorites preserve evidence of processes on their parent bodies, including alteration, metamorphism, and shock events. Here we show that the Kakowa (L6) ordinary chondrite (OC) preserves both shock-melt veins and pockets of detrital grains from a brecciat ... Full text Cite

Inverse methods for consistent quantification of seafloor anoxia using uranium isotope data from marine sediments

Journal Article Earth and Planetary Science Letters · January 1, 2022 Uranium isotopes (δ238U) have quickly become one of the most widely-used redox proxies in paleoceanographic studies. The quantitative power of the δ238U proxy derives from the long marine residence time of uranium and the dominance of reduction in fraction ... Full text Cite

The Sedimentary Geochemistry and Paleoenvironments Project.

Journal Article Geobiology · November 2021 Full text Cite

Mercury abundance and isotopic composition indicate subaerial volcanism prior to the end-Archean "whiff" of oxygen.

Journal Article Proceedings of the National Academy of Sciences of the United States of America · August 2021 Earth's early atmosphere witnessed multiple transient episodes of oxygenation before the Great Oxidation Event 2.4 billion years ago (Ga) [e.g., A. D. Anbar et al., Science 317, 1903-1906 (2007); M. C. Koehler, R. Buick, M. E. Barley, Precambrian ... Full text Cite

Benthic redox conditions and nutrient dynamics in the ca. 2.1 Ga Franceville sub-basin

Journal Article Precambrian Research · July 15, 2021 The co-existence of motile macroorganisms and mat-building cyanobacteria in the Paleoproterozoic FB2 Member of the Franceville sub-basin, Gabon, points to the possible emergence of multi-trophic-level biological interaction by 2.1 billion years (Ga) ago. H ... Full text Cite

Carbon cycle inverse modeling suggests large changes in fractional organic burial are consistent with the carbon isotope record and may have contributed to the rise of oxygen.

Journal Article Geobiology · July 2021 Abundant geologic evidence shows that atmospheric oxygen levels were negligible until the Great Oxidation Event (GOE) at 2.4-2.1 Ga. The burial of organic matter is balanced by the release of oxygen, and if the release rate exceeds efficient oxygen sinks, ... Full text Cite

High Organic Burial Efficiency Is Required to Explain Mass Balance in Earth's Early Carbon Cycle

Journal Article Global Biogeochemical Cycles · February 1, 2021 Earth's carbon cycle maintains a stable climate and biosphere on geological timescales. Feedbacks regulate the size of the surface carbon reservoir, and on million-year timescales the carbon cycle must be in steady state. A major question about the early E ... Full text Cite

Basinal hydrographic and redox controls on selenium enrichment and isotopic composition in Paleozoic black shales

Journal Article Geochimica et Cosmochimica Acta · October 15, 2020 Mass-dependent variations in selenium stable isotope ratios have recently been developed as a paleo-redox proxy. Since the reduction of selenium oxyanions occurs at a relatively high redox potential, this system holds promise for probing conditions relevan ... Full text Cite

Selenium isotope paleobiogeochemistry

Book · October 8, 2020 The attraction of selenium isotopes as a paleoenvironmental tracer lies in the high redox potential of selenium oxyanions (SeIV and SeVI), the two dominant species in the modern ocean. The largest isotopic fractionations occur during oxyanion reduction, wh ... Full text Cite

Redox fluctuations, trace metal enrichment and phosphogenesis in the ~2.0 Ga Zaonega Formation

Journal Article Precambrian Research · July 1, 2020 The ~2.0 Ga Zaonega Formation (ZF) holds one of the oldest phosphorites in the geologic record, reaching >15% P2O5. Understanding the depositional conditions that enabled sedimentary phosphorus enrichment in this unit will thus help us to interpret the sig ... Full text Cite

Exploring cycad foliage as an archive of the isotopic composition of atmospheric nitrogen.

Journal Article Geobiology · March 2020 Molecular nitrogen (N2 ) constitutes the majority of Earth's modern atmosphere, contributing ~0.79 bar of partial pressure (pN2 ). However, fluctuations in pN2 may have occurred on 107 -109 year times ... Full text Cite

Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era

Journal Article Earth and Planetary Science Letters · October 15, 2018 Nitrogen isotope ratios in marine sedimentary rocks have become a widely used biogeochemical proxy that records information about nutrient cycling and redox conditions in Earth's distant past. While the past two decades have seen considerable progress in o ... Full text Cite

Transient surface ocean oxygenation recorded in the ∼2.66-Ga Jeerinah Formation, Australia.

Journal Article Proceedings of the National Academy of Sciences of the United States of America · July 2018 Many paleoredox proxies indicate low-level and dynamic incipient oxygenation of Earth's surface environments during the Neoarchean (2.8-2.5 Ga) before the Great Oxidation Event (GOE) at ∼2.4 Ga. The mode, tempo, and scale of these redox changes are poorly ... Full text Cite

Biomass recycling and Earth's early phosphorus cycle.

Journal Article Science advances · November 2017 Phosphorus sets the pace of marine biological productivity on geological time scales. Recent estimates of Precambrian phosphorus levels suggest a severe deficit of this macronutrient, with the depletion attributed to scavenging by iron minerals. We propose ... Full text Cite

Spatial and temporal trends in Precambrian nitrogen cycling: A Mesoproterozoic offshore nitrate minimum

Journal Article Geochimica et Cosmochimica Acta · February 1, 2017 Fixed nitrogen is an essential nutrient for eukaryotes. As N2 fixation and assimilation of nitrate are catalyzed by metalloenzymes, it has been hypothesized that in Mesoproterozoic oceans nitrate was limited in offshore environments by low trace metal conc ... Full text Cite

Selenium isotopes record extensive marine suboxia during the Great Oxidation Event.

Journal Article Proceedings of the National Academy of Sciences of the United States of America · January 2017 It has been proposed that an "oxygen overshoot" occurred during the early Paleoproterozoic Great Oxidation Event (GOE) in association with the extreme positive carbon isotopic excursion known as the Lomagundi Event. Moreover, it has also been suggested tha ... Full text Cite

Modeling pN₂ through Geological Time: Implications for Planetary Climates and Atmospheric Biosignatures.

Journal Article Astrobiology · December 2016 Nitrogen is a major nutrient for all life on Earth and could plausibly play a similar role in extraterrestrial biospheres. The major reservoir of nitrogen at Earth's surface is atmospheric N2, but recent studies have proposed that the size of th ... Full text Cite

The evolution of Earth's biogeochemical nitrogen cycle

Journal Article Earth-Science Reviews · September 1, 2016 Nitrogen is an essential nutrient for all life on Earth and it acts as a major control on biological productivity in the modern ocean. Accurate reconstructions of the evolution of life over the course of the last four billion years therefore demand a bette ... Full text Cite