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.
Key Points
Upper ocean oxygenation levels are highly dynamic across OAE 2
A shallow O2 oasis in proto‐Atlantic is supported by proxy and model
I/Ca is a proxy for local redox, not for weathering and carbon burial
The methane (CH4) cycle is a key component of the Earth system that links planetary climate, biological metabolism, and the global biogeochemical cycles of carbon, oxygen, sulfur, and hydrogen. ...However, currently lacking is a numerical model capable of simulating a diversity of environments in the ocean, where CH4 can be produced and destroyed, and with the flexibility to be able to explore not only relatively recent perturbations to Earth's CH4 cycle but also to probe CH4 cycling and associated climate impacts under the very low-O2 conditions characteristic of most of Earth's history and likely widespread on other Earth-like planets. Here, we present a refinement and expansion of the ocean–atmosphere CH4 cycle in the intermediate-complexity Earth system model cGENIE, including parameterized atmospheric O2–O3–CH4 photochemistry and schemes for microbial methanogenesis, aerobic methanotrophy, and anaerobic oxidation of methane (AOM). We describe the model framework, compare model parameterizations against modern observations, and illustrate the flexibility of the model through a series of example simulations. Though we make no attempt to rigorously tune default model parameters, we find that simulated atmospheric CH4 levels and marine dissolved CH4 distributions are generally in good agreement with empirical constraints for the modern and recent Earth. Finally, we illustrate the model's utility in understanding the time-dependent behavior of the CH4 cycle resulting from transient carbon injection into the atmosphere, and we present model ensembles that examine the effects of atmospheric pO2, oceanic dissolved SO42-, and the thermodynamics of microbial metabolism on steady-state atmospheric CH4 abundance. Future model developments will address the sources and sinks of CH4 associated with the terrestrial biosphere and marine CH4 gas hydrates, both of which will be essential for comprehensive treatment of Earth's CH4 cycle during geologically recent time periods.
The concentration of CO2 in the atmosphere is sensitive to changes in the depth at which sinking particulate organic matter is remineralized: often described as a change in the exponent “b” of the ...Martin curve. Sediment trap observations from deep and intermediate depths suggest there is a spatially heterogeneous pattern of b, particularly varying with latitude, but disagree over the exact spatial patterns. Here we use a biogeochemical model of the phosphorus cycle coupled with a steady-state representation of ocean circulation to explore the sensitivity of preformed phosphate and atmospheric CO2 to spatial variability in remineralization depths. A Latin hypercube sampling method is used to simultaneously vary the Martin curve independently within 15 different regions, as a basis for a regression-based analysis used to derive a quantitative measure of sensitivity. Approximately 30 % of the sensitivity of atmospheric CO2 to changes in remineralization depths is driven by changes in the subantarctic region (36 to 60∘ S) similar in magnitude to the Pacific basin despite the much smaller area and lower export production. Overall, the absolute magnitude of sensitivity is controlled by export production, but the relative spatial patterns in sensitivity are predominantly constrained by ocean circulation pathways. The high sensitivity in the subantarctic regions is driven by a combination of high export production and the high connectivity of these regions to regions important for the export of preformed nutrients such as the Southern Ocean and North Atlantic. Overall, regionally varying remineralization depths contribute to variability in CO2 of between around 5 and 15 ppm, relative to a global mean change in remineralization depth. Future changes in the environmental and ecological drivers of remineralization, such as temperature and ocean acidification, are expected to be most significant in the high latitudes where CO2 sensitivity to remineralization is also highest. The importance of ocean circulation pathways to the high sensitivity in subantarctic regions also has significance for past climates given the importance of circulation changes in the Southern Ocean.
Variations in sedimentation rate, bioturbation, winnowing, and dissolution modify the deep‐sea sedimentary record, complicating the apparent relationship between stratigraphic depth and time of a ...geochemical proxy record and confounding the extraction of a clear picture of past climates and environments. Dynamic time warping (DTW) is used to align time series with similar patterns. Here we explore the use of DTW to identify gaps in proxy records of the Paleocene‐Eocene thermal maximum (PETM), aligning bulk sediment carbonate isotope records (δ13C) from various deep‐sea sediment core sections spanning the event. Alignment of PETM δ13C records from the Walvis Ridge, South Atlantic transect of ODP Leg 208 (Sites 1262, 1263, and 1265) was similar to previously published manually established alignments and consistent with the expectation that shallower sites have more complete records. The δ13C record from a Southern Ocean site (Maud Rise; ODP Site 690) was then aligned to ODP Site 1263, the most complete Walvis Ridge site. This alignment identifies a gap in Site 690, indicating that peak excursion δ13C values were not recorded. We conclude that DTW provides an objective way to align climate proxy records and rectify data loss associated with unconformities and other types of distortions, leading to a more complete understanding of the geologic record of past episodes of biotic and environmental change.
Key Points
Dynamic time warping can be employed to successfully align chemostratigraphic records
Dynamic time warping provides an objective way to align δ13C records of the PETM
The new alignments indicate gaps in sections previously considered complete, demonstrating the value of the approach
The response of the Earth system to greenhouse-gas-driven warming is of
critical importance for the future trajectory of our planetary environment.
Hyperthermal events – past climate transients with ...global-scale warming
significantly above background climate variability – can provide insights
into the nature and magnitude of these responses. The largest hyperthermal of
the Cenozoic was the Paleocene–Eocene Thermal Maximum (PETM ∼ 56 Ma).
Here we present new high-resolution bulk sediment stable isotope and major
element data for the classic PETM section at Zumaia, Spain. With these data we
provide a new detailed stratigraphic correlation to other key deep-ocean and
terrestrial PETM reference sections. With this new correlation and age model
we are able to demonstrate that detrital sediment accumulation rates within
the Zumaia continental margin section increased more than 4-fold during
the PETM, representing a radical change in regional hydrology that drove
dramatic increases in terrestrial-to-marine sediment flux. Most remarkable is
that detrital accumulation rates remain high throughout the body of the PETM,
and even reach peak values during the recovery phase of the characteristic
PETM carbon isotope excursion (CIE). Using a series of Earth system model
inversions, driven by the new Zumaia carbon isotope record, we demonstrate
that the silicate weathering feedback alone is insufficient to recover the
PETM CIE, and that active organic carbon burial is required to match the
observed dynamics of the CIE. Further, we demonstrate that the period of maximum organic
carbon sequestration coincides with the peak in detrital accumulation rates
observed at Zumaia. Based on these results, we hypothesise that orbital-scale
variations in subtropical hydro-climates, and their subsequent impact on
sediment dynamics, may contribute to the rapid climate and CIE recovery from
peak-PETM conditions.
The evolutionary success of planktic calcifiers during the Phanerozoic stabilized the climate system by introducing a new mechanism that acts to buffer ocean carbonate-ion concentration: the ...saturation-dependent preservation of carbonate in sea-floor sediments. Before this, buffering was primarily accomplished by adjustment of shallow-water carbonate deposition to balance oceanic inputs from weathering on land. Neoproterozoic ice ages of near-global extent and multimillion-year duration and the formation of distinctive sedimentary (cap) carbonates can thus be understood in terms of the greater sensitivity of the Precambrian carbon cycle to the loss of shallow-water environments and$CO_{2}-climate$feedback on ice-sheet growth.
Understanding the response of the Earth's climate system to anthropogenic perturbation has been a pressing priority for society since the late 1980s. However, recent years have seen a major paradigm ...shift in how such an understanding can be reached. Climate change demands analysis within an integrated 'Earth-system' framework, taken to encompass the suite of interacting physical, chemical, biological and human processes that, in transporting and transforming materials and energy, jointly determine the conditions for life on the whole planet. This is a highly complex system, characterized by multiple nonlinear responses and thresholds, with linkages often between apparently disparate components. The interconnected nature of the Earth system is wonderfully illustrated by the diverse roles played by atmospheric transport of mineral 'dust', particularly in its capacity as a key pathway for the delivery of nutrients essential to plant growth, not only on land, but perhaps more importantly, in the ocean. Dust therefore biogeochemically links land, air and sea. This paper reviews the biogeochemical role of mineral dust in the Earth system and its interaction with climate, and, in particular, the potential importance of both past and possible future changes in aeolian delivery of the micro-nutrient iron to the ocean. For instance, if, in the future, there was to be a widespread stabilization of soils for the purpose of carbon sequestration on land, a reduction in aeolian iron supply to the open ocean would occur. The resultant weakening of the oceanic carbon sink could potentially offset much of the carbon sequestered on land. In contrast, during glacial times, enhanced dust supply to the ocean could have 'fertilized' the biota and driven atmospheric CO2 lower. Dust might even play an active role in driving climatic change; since changes in dust supply may affect climate, and changes in climate, in turn, influence dust, a 'feedback loop' is formed. Possible feedback mechanisms are identified, recognition of whose operation could be crucial to our understanding of major climatic transitions over the past few million years.
During the four most recent glacial cycles, atmospheric CO2 during glacial maxima has been lowered by about 90–100 ppm with respect to interglacials. There is widespread consensus that most of this ...carbon was partitioned in the ocean. It is, however, still debated which processes were dominant in achieving this increased carbon storage. In this paper, we use an Earth system model of intermediate complexity to explore the sensitivity of ocean carbon storage to ocean circulation state. We carry out a set of simulations in which we run the model to pre-industrial equilibrium, but in which we achieve different states of ocean circulation by changing forcing parameters such as wind stress, ocean diffusivity and atmospheric heat diffusivity. As a consequence, the ensemble members also have different ocean carbon reservoirs, global ocean average temperatures, biological pump efficiencies and conditions for air–sea CO2 disequilibrium. We analyse changes in total ocean carbon storage and separate it into contributions by the solubility pump, the biological pump and the CO2 disequilibrium component. We also relate these contributions to differences in the strength of the ocean overturning circulation. Depending on which ocean forcing parameter is tuned, the origin of the change in carbon storage is different. When wind stress or ocean diapycnal diffusivity is changed, the response of the biological pump gives the most important effect on ocean carbon storage, whereas when atmospheric heat diffusivity or ocean isopycnal diffusivity is changed, the solubility pump and the disequilibrium component are also important and sometimes dominant. Despite this complexity, we obtain a negative linear relationship between total ocean carbon and the combined strength of the northern and southern overturning cells. This relationship is robust to different reservoirs dominating the response to different forcing mechanisms. Finally, we conduct a drawdown experiment in which we investigate the capacity for increased carbon storage by artificially maximising the efficiency of the biological pump in our ensemble members. We conclude that different initial states for an ocean model result in different capacities for ocean carbon storage due to differences in the ocean circulation state and the origin of the carbon in the initial ocean carbon reservoir. This could explain why it is difficult to achieve comparable responses of the ocean carbon pumps in model inter-comparison studies in which the initial states vary between models. We show that this effect of the initial state is quantifiable. The drawdown experiment highlights the importance of the strength of the biological pump in the control state for model studies of increased biological efficiency.
The Phanerozoic has seen fundamental changes in the global biogeochemical cycling of calcium carbonate (CaCO
3), particularly the advent of biomineralization during the early Cambrian when the ...products of weathering could first be removed through metabolic expenditure, and the subsequent ecological success of planktic calcifiers during the Mesozoic which shifted the locus of deposition from the continental shelves to the deep open ocean. These biologically-driven CaCO
3 depositional ‘mode’ changes along with geochemical and tectonic variations in boundary conditions such as sea-level and calcium ion concentrations all affected the carbonate chemistry of the ocean. I employ a model of atmosphere–ocean–sediment carbon cycling to explore the impact of these factors on the saturation state and carbonate chemistry of the global ocean during the Phanerozoic.
The model results highlight that overall; the time evolution and regulation of Phanerozoic ocean chemistry are dominated by a Mid Mesozoic Revolution in the marine carbonate cycle. Prior to this transition, it was possible for the ocean to attain states of extreme saturation during the Permian and Triassic as well as during the late Precambrian. This is primarily a consequence of low sea-level in restricting the potential area for the deposition of shallow water carbonates, thus requiring a more saturated ocean and higher rate of precipitation per unit area in order to balance weathering input. This is consistent with the occurrence of mineralogically ‘anomalous’ carbonates during these periods but not commonly at other times. That the modern carbon cycle does not respond to similar tectonic forcings is due to the ecological success of calcifying planktic taxa during the Mesozoic, which in facilitating the creation of a responsive deep-sea carbonate sink enabled a much greater degree of regulation of saturation state than was previously possible. The model results also highlight the primary role of changes in the concentration of CO
2 in the atmosphere and of Ca
2+ in the ocean in determining surface
pH. The uncertainty inherent in paleo CO
2 estimates then translates into sufficient uncertainty in reconstructions of Phanerozoic temperature variability that one can only deduce from the carbonate δ
18O record that the Cretaceous was generally warmer and the Carboniferous colder than average. The substantially enhanced oceanic carbon inventory predicted for the Paleozoic suggests that previous calculations of methane hydrate release may have substantially underestimated the quantity of clathrate carbon required to explain observed carbon isotopic excursions. In both cases the importance of quantifying Phanerozoic marine chemistry is clear.
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