Dalton S. Hardisty

An iodine record of Paleoproterozoic surface ocean oxygenation

Constraining oxygen levels in the early Precambrian surface ocean has been a longstanding goal, but efforts have been challenged by the availability of suitable proxies. Here we present a novel approach, iodine geochemistry, which broadens our perspective by providing constraints on shallow, carbonate-dominated marine settings. Iodate (IO 3 – ) persists exclusively in oxic waters and is the sole iodine species incorporated into carbonate minerals, allowing iodine-to-calcium ratios (I/Ca) in shallow carbonates to be used as a paleoredox indicator. Our data from a series of Mesoarchean through Paleoproterozoic carbonates deposited under shallow-marine conditions reveal a progressive surface ocean oxygenation in the early Paleoproterozoic. These data seem to indicate that a largely anoxic surface ocean extended throughout the Archean until the Great Oxidation Event (GOE) at ca. 2.4 Ga, implying that previous inferences of pre-GOE oxygen production may reflect oxygen oases, transient oxidation events, or oxygen levels below those required for IO 3 – accumulation. The data suggest formation and persistence of IO 3 – and, consequently, surface ocean oxygen concentrations of at least 1 μM during the GOE. Following the initial rise of oxygen, carbonate-associated iodine in globally extensive carbonate units deposited during the Lomagundi positive carbon isotope excursion at ca. 2.22–2.1 Ga suggests a widespread aerobic iodine cycle beyond that operating prior to the event, synchronous with high relative rates of organic carbon burial and apparent expansion of oxidative conditions.

Upper ocean oxygenation dynamics from I/Ca ratios during the Cenomanian-Turonian OAE 2

Global warming lowers the solubility of gases in the ocean and drives an enhanced hydrological cycle with increased nutrient loads delivered to the oceans, leading to increases in organic production, the degradation of which causes a further decrease in dissolved oxygen. In extreme cases in the geological past, this trajectory has led to catastrophic marine oxygen depletion during the so-called oceanic anoxic events (OAEs). How the water column oscillated between generally oxic conditions and local/global anoxia remains a challenging question, exacerbated by a lack of sensitive redox proxies, especially for the suboxic window. To address this problem, we use bulk carbonate I/Ca to reconstruct subtle redox changes in the upper ocean water column at seven sites recording the Cretaceous OAE 2. In general, I/Ca ratios were relatively low preceding and during the OAE interval, indicating deep suboxic or anoxic waters exchanging directly with near-surface waters. However, individual sites display a wide range of initial values and excursions in I/Ca through the OAE interval, reflecting the importance of local controls and suggesting a high spatial variability in redox state. Both I/Ca and an Earth System Model suggest that the northeast proto-Atlantic had notably higher oxygen levels in the upper water column than the rest of the North Atlantic, indicating that anoxia was not global during OAE 2 and that important regional differences in redox conditions existed. A lack of correlation with calcium, lithium, and carbon isotope records suggests that neither enhanced global weathering nor carbon burial was a dominant control on the I/Ca proxy during OAE 2.

Uncovering the spatial heterogeneity of Ediacaran carbon cycling

Records of the Ediacaran carbon cycle (635-541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to -12‰). The nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ13 C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ13 Ccarb and δ13 Corg ) records from the Ediacaran Doushantuo Formation along a proximal-to-distal transect across the Yangtze Platform of South China as a test of the spatial variation of the SE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ13 Corg are highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that the SE is a primary marine signature. Data compilations demonstrate that the SE was also accompanied globally by parallel negative shifts of δ34 S of carbonate-associated sulfate (CAS) and increased 87 Sr/86 Sr ratio and coastal CAS concentration, suggesting elevated continental weathering and coastal marine sulfate concentration during the SE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of the SE and coexisting δ13 Corg records of the Doushantuo, with likely relevance to the SE in other regions. In this model, we infer continued marine redox stratification through the SE but with increased availability of oxidants (e.g., O2 and sulfate) limited to marginal near-surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving the SE, future models must consider the evidence for spatial heterogeneity in δ13 C presented in this study.

A Holocene history of dynamic water column redox conditions in the Landsort Deep, Baltic Sea

The modern Baltic Sea is the worlds largest anthropogenically forced anoxic basin. Using integrated geochemical records collected during Integrated Ocean Drilling Program (IODP) Expedition 347 from the deepest and one of the most reducing sub-basins in the Baltic Sea, Landsort Deep, we explore the degree and frequency of natural anoxia through the Baltic Holocene. A marked decrease in carbon-to-sulfur ratios (C/S) from the cores indicate the transition from the Baltic Ice Lake to the current brackish sea, which occurred about 8.5 kyrs B.P. Following this, laminations throughout sediments recording brackish deposition suggest sustained anoxia or extreme low oxygen, while high molybdenum (Mo) concentrations of >100 ppm and iron (Fe) geochemistry suggest water column sulfide accumulation, or euxinia, that persisted beyond seasonal timescales during deposition of two distinct sapropel units. Sedimentary Mo isotope values range from +1.11 to −0.50 permil, which are distinctly fractionated from modern Baltic seawater (+2.26 to +2.67‰) and thus indicate that each of the sapropels experienced only weak and/or oscillatory euxinia—in contrast to the more stable euxinic conditions of more restricted basins. A shift in δ98Mo starting above the lower sapropel to a distinctly more negative range suggests particularly weak and oscillatory euxinia, with an enhanced contribution of manganese (Mn) redox cycling to Mo deposition relative to the lower portion of the profile. This conclusion is supported by extreme sedimentary Mn enrichments of up to 15 weight percent. We interpret the combined data to indicate episodic but major Baltic inflow events of saline and oxygenated North Sea water into the anoxic Landsort Deep that limited the concentrations and residence time of water column sulfide and caused episodic oxide deposition. Considering the temporal overlap between the most reducing conditions and periods of redox instability, we hypothesize that major Baltic inflows, as is observed today, lead to short-term instability while simultaneously supporting longer-term Baltic anoxia by strengthening the halocline. Ultimately, our results indicate that periods more reducing than the modern Baltic Sea have occurred naturally over the Holocene, but the characteristic dynamic saline inputs have historically prevented the relatively more widespread and stable anoxia observed in other classic restricted basins and will likely continue to do so.

Late inception of a resiliently oxygenated upper ocean

The rise of oxygen To understand the evolution of the biosphere, we need to know how much oxygen was present in Earths atmosphere during most of the past 2.5 billion years. However, there are few proxies sensitive enough to quantify O2 at the low levels present until slightly less than 1 billion years ago. Lu et al. measured iodine/calcium ratios in marine carbonates, which are a proxy for dissolved oxygen concentrations in the upper ocean. They found that a major, but temporary, rise in atmospheric O2 occurred at around 400 million years ago and that O2 levels underwent a step change to near-modern values around 200 million years ago. Science, this issue p. 174 The I/Ca ratio in marine carbonates tracks atmospheric oxygen levels for the past 2.5 billion years. Rising oceanic and atmospheric oxygen levels through time have been crucial to enhanced habitability of surface Earth environments. Few redox proxies can track secular variations in dissolved oxygen concentrations around threshold levels for metazoan survival in the upper ocean. We present an extensive compilation of iodine-to-calcium ratios (I/Ca) in marine carbonates. Our record supports a major rise in the partial pressure of oxygen in the atmosphere at ~400 million years (Ma) ago and reveals a step change in the oxygenation of the upper ocean to relatively sustainable near-modern conditions at ~200 Ma ago. An Earth system model demonstrates that a shift in organic matter remineralization to greater depths, which may have been due to increasing size and biomineralization of eukaryotic plankton, likely drove the I/Ca signals at ~200 Ma ago.

The iron paleoredox proxies: A guide to the pitfalls, problems and proper practice

Oceanic anoxia—including euxinic settings defined by the presence of water column hydrogen sulfide (H2S)—is minor in the ocean today. Such conditions, however, were common or even dominant in the past, particularly during the Precambrian and Phanerozoic oceanic anoxic events. The latter are associated with massive petroleum and mineral reserves and many of the major extinction events in the paleontological record. Our ability to recognize ancient oxygen deficiencies relies strongly on paleontological data viewed in combination with geochemical tracers, and geochemistry is typically our only window onto ancient marine redox during the Precambrian when diagnostic skeletal and behaviorial traces of oxygen-dependent animals are mostly missing. So far no approach has gained wider acceptance than the iron proxies, which rely generally on quantification of the extent to which reactive iron (as oxides principally) is converted to pyrite. The promise of these approaches lies in part with the relative ease of measurement, but it is this ease and the corresponding widespread use that has also led to misuses. Much of the recent confidence in the iron paleoredox proxies lies with sophisticated deconstruction of the reactive Fe pool via mineral-calibrated wet chemical speciation. These validations and calibrations, mostly in the modern ocean, expose the challenges, while at the same time opening other doors of opportunity as the catalog of controlling factors extends beyond water column redox to include sedimentation rate, sedimentary Fe remobilization, signals of oscillatory redox, and hydrothermal versus other primary Fe inputs to the ocean, among other factors. Also key is a deep understanding of the limitations imposed—or at least the due diligence required—as linked to mineral transformations during burial and metamorphism. This review seeks to highlight many of the key issues, including appropriate sample choices, as a roadmap for those keen to apply Fe proxies in their studies of ancient oceans and their relationships to co-evolving life. Among the critical messages to take away is the value of robust Fe-based measures of local redox that, when combined with elemental mass balances and isotopic proxies dependent on those local conditions, can shed light on the global redox state of the oceans through time and related implications for the history of life on Earth.

An evaluation of sedimentary molybdenum and iron as proxies for pore fluid paleoredox conditions

Iron speciation and trace metal proxies are commonly applied together in efforts to identify anoxic settings marked by the presence of free sulfide (euxinia) or dissolved iron (ferruginous) in the water column. Here, we use a literature compilation from modern localities to provide a new empirical evaluation of coupled Fe speciation and Mo concentrations as a proxy for pore water sulfide accumulation at non-euxinic localities. We also present new Fe speciation, Mo concentration, and S isotope data from the Friends of Anoxic Mud (FOAM) site in Long Island Sound, which is marked by pore water sulfide accumulation of up to 3 mM beneath oxygen-containing bottom waters. For the operationally defined Fe speciation scheme, ‘highly reactive’ Fe (FeHR) is the sum of pyritized Fe (Fepy) and Fe dominantly present in oxide phases that is available to react with pore water sulfide to form pyrite. Observations from FOAM and elsewhere confirm that Fepy/FeHR from non-euxinic sites is a generally reliable indicator of pore fluid redox, particularly the presence of pore water sulfide. Molybdenum (Mo) concentration data for anoxic continental margin sediments underlying oxic waters but with sulfidic pore fluids typically show authigenic Mo enrichments (2–25 ppm) that are elevated relative to the upper crust (1–2 ppm). However, compilations of Mo concentrations comparing sediments with and without sulfidic pore fluids underlying oxic and low oxygen (non-euxinic) water columns expose non-unique ranges for each, exposing false positives and false negatives. False positives are most frequently found in sediments from low oxygen water columns (for example, Peru Margin), where Mo concentration ranges can also overlap with values commonly found in modern euxinic settings. FOAM represents an example of a false negative, where, despite elevated pore water sulfide concentrations and evidence for active Fe and Mn redox cycling in FOAM sediments, sedimentary Mo concentrations show a homogenous vertical profile across 50 cm depth at 1 to 2 ppm. A diagenetic model for Mo provides evidence that muted authigenic enrichments are derived from elevated sedimentation rates. Consideration of a range of additional parameters, most prominently pore water Mo concentration, can replicate the ranges of most sedimentary Mo concentrations observed in modern non-euxinic settings. Together, the modern Mo and Fe data compilations and diagenetic model provide a framework for identifying paleo-pore water sulfide accumulation in ancient settings and linked processes regulating seawater Mo and sulfate concentrations and delivery to sediments. Among other utilities, identifying ancient accumulation of sulfide in pore waters, particularly beneath oxic bottom waters, constrains the likelihood that those settings could have hosted organisms and ecosystems with thiotrophy at their foundations.