With the advent of computationally fast “intermediate complexity” models of Earth's climate and carbon cycle the marine record can be interpreted much more directly than before. Specifically, the ...mechanistic simulation of deep‐sea sediment cores provides an important step in bridging the model‐data divide. Here I use this methodology to help interpret the excursion in carbonate compensation depth (CCD) during the Paleocene‐Eocene thermal maximum that is recorded in cores recovered from Walvis Ridge in the South Atlantic. By explicitly simulating the expected geological record of a massive CO2 release in an Earth system model I show how reduction in the intensity of subsurface mixing of the sediment by benthic animals substantially magnifies the recorded shoaling of the CCD. Conversely, to quantify correctly the carbon release consistent with the observed CCD changes, one must account for any bioturbational changes. A reduction in sediment mixing intensity also appears to be important to reproducing the sharpness of the contact between carbonate‐rich late Paleocene sediments and the overlying early Eocene clay layer at Walvis Ridge. Assuming a relatively rapid (approximately 1 ka duration) CO2 release further helps to account for some of the paleoceanographic observations. Finally, interbasin differences in bioturbational regime help resolve some of the observed disparity in carbonate preservation between Walvis Ridge and sites outside of the Atlantic, although changes in ocean ventilation and circulation are also likely to play a critical role in this.
An end to the “rain ratio” reign? Ridgwell, Andy J.
Geochemistry, geophysics, geosystems : G3,
June 2003, Letnik:
4, Številka:
6
Journal Article
Recenzirano
Odprti dostop
One of the most elegant mechanisms forwarded for late Quaternary atmospheric CO2 variability concerns the sensitivity of calcium carbonate preservation in deep ocean sediments to the relative ...delivery rates of calcium carbonate and particulate organic carbon (the CaCO3:POC “rain ratio”). It was implicitly assumed that any change in the CaCO3:POC rain ratio of biogenic material produced in the surface ocean will be communicated directly to the sediments. This would allow relatively subtle shifts in ecosystem composition to affect sedimentary CaCO3 preservation (and thus atmospheric CO2). However, recent research into the controls on the transport of POC to depth suggests that the rain ratio “seen” by the sediments may instead be buffered against any perturbation occurring at the surface. This casts doubt on the viability of hypotheses envisaging ecological changes as a means of accounting for the observed glacial‐interglacial CO2 signal.
As an episode of rapid global warming associated with the release of massive quantities of carbon to the atmosphere and oceans, the Paleocene–Eocene Thermal Maximum (PETM, ∼56 Ma) is considered a ...potential analog for modern anthropogenic carbon emissions. However, the prevailing order of magnitude uncertainty in the rate of carbon release during the PETM precludes any straightforward comparison between the paleo-record and the modern. Similar barriers exist to the interpretation of many other carbon isotope excursions in the geological record. Here we use the Earth system model cGENIE to quantify the consequences of differing carbon emissions rates on the isotopic record of different carbon reservoirs. We explore the consequences of a range of emissions scenarios – from durations of carbon input of years to millennia and constant versus pulsed emissions rates, and trace how the isotopic signal is imprinted on the different carbon reservoirs. From this, we identify a characteristic relationship between the difference in carbon isotope excursion sizes between atmospheric CO2 and dissolved inorganic carbon (DIC) and the duration of carbon emissions. To the extent that available isotopic data spanning the PETM constrain the size of the marine and atmospheric carbon isotopic excursions, applying this empirical relationship suggests the duration of the component of carbon emissions that dominates the isotopic signal could be less than 3000 yr. However, utilizing the ratio of excursion size in the atmosphere to ocean as a metric to constrain duration of carbon emissions highlights the necessity to strengthen estimates for these two measurements across the PETM. Our general interpretive framework could be equally applied in assessing rates of carbon emissions for other geological events.
•cGENIE Earth system modeling of abrupt carbon release.•New empirical relationship suggests PETM carbon release could be less than 3000 yr.•Bomb radiocarbon poor analogy for past δ13C excursions.•Framework for interpreting rates of geological δ13C excursions.
Variations in carbon‐14 to carbon‐12 ratio in the atmosphere (Δ14Catm) provide a powerful diagnostic for elucidating the timing and nature of geophysical and anthropological change. The (Atlantic) ...marine archive suggests a rapid Δ14Catm increase of 50‰ at the onset of the Younger Dryas (YD) cold reversal (12.9–11.7 kyr BP), which has not yet been satisfactorily explained in terms of magnitude or causal mechanism, as either a change in ocean ventilation or production rate. Using Earth‐system model simulations and comparison of marine‐based radiocarbon records from different ocean basins, we demonstrate that the YD Δ14Catm increase is smaller than suggested by the marine archive. This is due to changes in reservoir age, predominantly caused by reduced ocean ventilation.
There is international consensus that 'dangerous' climate change must be avoided. Yet without radical changes in energy sources and usage and global economies, changes that so far society has been ...unable or unwilling to make, it seems highly likely that we will start to experience unacceptably damaging and/or societally disruptive global environmental change later this century. What actions can be taken to safeguard future environmental quality, ecosystems, agriculture, economy, and society? A new science- 'geoengineering'- that until recently would have seemed pure science fiction, promises an alternative way of temporarily regaining control of climate. Colossal engineering schemes to shade the sun, make the atmosphere hazier, modify clouds, even throw iron into the ocean, are all being promoted as possible ways out of our dilemma. This article considers the state of this new science, and its implications for society.
The environmental conditions of Earth, including the climate, are determined by physical, chemical, biological, and human interactions that transform and transport materials and energy. This is the ..."Earth system": a highly complex entity characterized by multiple nonlinear responses and thresholds, with linkages between disparate components. One important part of this system is the iron cycle, in which iron-containing soil dust is transported from land through the atmosphere to the oceans, affecting ocean biogeochemistry and hence having feedback effects on climate and dust production. Here we review the key components of this cycle, identifying critical uncertainties and priorities for future research.
We review one of the most ancient of all the global biogeochemical cycles and one which reflects the profound geochemical and biological changes that have occurred as the Earth system has evolved ...through time—that of calcium carbonate (CaCO
3). In particular, we highlight a Mid-Mesozoic Revolution in the nature and location of carbonate deposition in the marine environment, driven by the ecological success of calcareous plankton. This drove the creation of a responsive deep-sea sedimentary sink of CaCO
3. The result is that biologically driven carbonate deposition provides a significant buffering of ocean chemistry and of atmospheric CO
2 in the modern system. However, the same calcifying organisms that under-pin the deep-sea carbonate sink are now threatened by the continued atmospheric release of fossil fuel CO
2 and increasing acidity of the surface ocean. We are not yet in a position to predict what the impact on CaCO
3 production will be, or how the uptake of fossil fuel CO
2 by the ocean will be affected. This uncertainty in the future trajectory of atmospheric CO
2 that comes from incomplete understanding of the marine carbonate cycle is cause for concern.
The “iron hypothesis” posits a role for increased supply of mineral aerosol to the ocean surface during glacial periods in driving atmospheric CO2 lower; that changes in CO2 and climate strongly ...affect dust supply raises the possibility of feedback. Here I take a systems view in analyzing the properties and implications of such a feedback and consider three primary state variables that can be related empirically to each other: dust supply, atmospheric CO2, and “climate” (surface air temperature). The results of this analysis suggest that the dust‐CO2‐climate feedback is primarily an intraglacial phenomenon, when it can account for about a third of the temperature variability recorded in Antarctic ice cores. Since glacial‐interglacial cyclicity prior to ca. 800 kyr BP is characterized by the absence of a “full” glacial state (such as the Last Glacial Maximum), it is possible that destabilization of climate by the marine iron cycle is fundamental to the differences between “41 kyr” and “100 kyr” climatic regimes. The critical role played by the state of the land surface in this feedback also has implications for the longer‐term evolution of the Earth system during the Cenozoic.
On geological time-scales, the production and degree of recycling of biogenic carbon in the marine realm and ultimately its removal to sediments, exerts a dominant control on atmospheric CO2 and ...hence variability in climate. This is a highly complex system involving a myriad of inter-connected biological, chemical, and physical processes. For this reason alone, linking observations, often highly abstracted in the form of proxies, to the primary processes involved and ultimately to explanatory hypotheses for specific geological events and transitions, is challenging. The past few decades have seen a progressive improvement in theoretical and process-based understanding of the various components that make up the marine carbon cycle and, hand-in-hand with this, the development of numerical model representations of the complete system. Models have also been designed and/or adapted with paleoclimate questions in mind and applied to quantitatively explore the role of the marine carbon cycle in both perturbations and long-term geologic evolutionary trends in global climate, and possible feedbacks between them. However, we must ask whether paleoclimate models incorporate sufficiently appropriate representations of the dynamics and sensitivities of the marine carbon cycle, and indeed, whether in the geological context, we really know what these dynamics are.
Here we provide a comprehensive overview of how marine carbon cycling and the biological carbon pump is treated in available paleoclimate models, with the aim of critically evaluating their ability to help interpret past marine carbon cycle and climate dynamics. To this end, we first provide an overview of commonly used paleoclimate models and some of their associated paleo-applications, drawing from a wide range of global carbon cycle box models and Earth system Models of Intermediate Complexity (EMICs). Secondly, we review and evaluate the three dominant processes involved in the cycling of organic and inorganic carbon in the marine system and how they are represented in models, namely: biological productivity at the ocean surface, remineralisation/dissolution of particulate carbon within the water column, and the benthic-pelagic coupling at the seafloor. We generate and employ illustrative examples using the model GENIE to show how different parameterisations of water-column and sediment processes can lead to significantly different model projections. Our compilation reveals the prevalence of static parameterisations of marine carbon cycling among existing paleoclimate models, which are commonly empirically derived from present-day observations. Although such approaches tend to represent carbon transfer in the modern ocean well, they are potentially compromised in their ability to reflect the true degree of freedom and strength of feedbacks with respect to past climate events, particularly those characterised by environmental boundary conditions that differ fundamentally from today. Finally, we discuss the importance of using models of different complexities and how questions of model uncertainty may start to be addressed.
Planktonic foraminifera are important calcifiers in the modern ocean. Despite this importance, the main functions of foraminifera's test and ornamentation such as spines are unclear. Spinose species ...dominate the planktonic foraminifera population in subtropical oligotrophic gyres, while non-spinose species dominate in deeper waters and at high latitudes suggesting that spines help foraminifera in food-limited areas. Here we take a novel approach to investigate the benefits of spines on foraminifera foraging using a 0-D trait-based ecosystem model. The model considers the traits of size, calcification, spines, passive feeding, and diet. We assess how the presence of spines impact foraminifera diet and fitness via a series of simulated environments representing oligo, meso- and eutrophic settings at different temperatures. We find that independent of diet, non-spinose taxa need to be more size-generalist predators than other zooplankton species to maintain their population. In contrast, spinose species benefit from a relatively higher surface-to-volume ratio compared to non-spinose species, which allows them to be as generalist as other zooplankton groups. In agreement with observations, we find that herbivory is the most successful diet in cold environments, while carnivory allows foraminifera to be more successful in warm environments.
•Non-spinose species need to be more generalist predators or omnivorous to survive.•Spinose species benefit from spines on resource competition.•Herbivory is more efficient in cold environments while carnivory in warm habitats.
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