Multi-proxy constraints on the significance of covariant δ13C values in carbonate and organic carbon during the early Mississippian

This study investigates the covariation between carbonate and organic δ13C values in a proximal to distal transect of four outcrops in the Madison Limestone in the Western United States Rockies, combined with δ34S values of carbonate associated sulphate, the concentration of acid-insoluble material and measurements of total organic carbon. These new geochemical datasets not only allow for an evaluation of carbon isotope covariance during one of the largest perturbations to the global carbon cycle over the past 550 Myr, but also constrain the cause of the excursion in carbonate δ13C values. The results support the hypothesis that a period of anoxia did not play a role in generating the positive carbonate δ13C values, but rather favour interpretations by previous workers that the proliferation of land plants destabilized the Carboniferous carbon cycle, setting the stage for a significant change in the carbonate δ13C values of contemporaneous marine carbonates. These results also demonstrate that one of the largest perturbations to the global carbon cycle did not produce synchronous variations in carbonate and organic δ13C values, emphasizing the importance of local depositional controls on carbon isotope covariance in the geological record in both modern and ancient environments

African biomass burning is a substantial source of phosphorus deposition to the Amazon, Tropical Atlantic Ocean, and Southern Ocean

The deposition of phosphorus (P) from African dust is believed to play an important role in bolstering primary productivity in the Amazon Basin and Tropical Atlantic Ocean (TAO), leading to sequestration of carbon dioxide. However, there are few measurements of African dust in South America that can robustly test this hypothesis and even fewer measurements of soluble P, which is readily available for stimulating primary production in the ocean. To test this hypothesis, we measured total and soluble P in long-range transported aerosols collected in Cayenne, French Guiana, a TAO coastal site located at the northeastern edge of the Amazon. Our measurements confirm that in boreal spring when African dust transport is greatest, dust supplies the majority of P, of which 5% is soluble. In boreal fall, when dust transport is at an annual minimum, we measured unexpectedly high concentrations of soluble P, which we show is associated with the transport of biomass burning (BB) from southern Africa. Integrating our results into a chemical transport model, we show that African BB supplies up to half of the P deposited annually to the Amazon from transported African aerosol. This observational study links P-rich BB aerosols from Africa to enhanced P deposition in the Amazon. Contrary to current thought, we also show that African BB is a more important source of soluble P than dust to the TAO and oceans in the Southern Hemisphere and may be more important for marine productivity, particularly in boreal summer and fall

The Significance of Covariance between the Carbon Isotopic Composition of Marine Carbonates and Organic Matter: Assumptions, Processes, and Applications

The global carbon cycle refers to a series of processes that transfer carbon between the various reservoirs on Earth, including the atmosphere, the oceans, the sediments, the biosphere, and mantle. Over geologic time scales, the transfer of carbon to and from the atmosphere via volcanism, continental weathering, and the deposition of carbonate sediments works to regulate the concentration of atmospheric CO2, which as a result has a profound influence upon the Earth’s climate. However, on time scales of less than a million years, these same processes can produce imbalances in the amount of carbon in a given reservoir. Such perturbations to the atmosphere, either in the rate of carbon transfer or a change in the source of carbon, are thought to be recorded in the carbon isotopic composition of both marine carbonates and marine organic material produced by photosynthetic organisms. Marine carbonate and organic carbon isotope values are considered some of the best records of changes in the rate of carbon cycling since they are produced in the surface waters of the ocean. Air-sea gas exchange at the interface between the oceans and atmosphere translates perturbations to the atmospheric reservoir to the surface ocean in equilibrium. As a result, deposits of marine carbonates and organic material are thought to directly record changes in the carbon isotope value of the atmosphere over geologic time scales. A common approach used to evaluate changes in the global carbon cycle is to analyze the relationship between carbon isotope values of co-occurring marine carbonates and organic material. The projects presented in this dissertation test the assumptions of this approach. The fundamental assumption is that carbonate and organic carbon isotope values which exhibit covariance, or simultaneous changes in their isotopic composition, are direct records of the carbon isotope value of the CO2 dissolved in the surface waters of the ocean (DIC). In fact, some significant biogeochemical events in Earth history are accompanied by covariant carbon isotope values in co-occurring marine carbonates and organic material, including the Permo-Triassic Boundary, some of the Cretaceous Ocean Anoxic Events, the Paleocene-Eocene Thermal Maximum, and even some of the Precambrian anomalies associated with Snowball Earth conditions. However, the assumption that covariance between carbonate and organic carbon isotope values record changes in the global carbon cycle has not been fully investigated in more recent periods in Earth history where changes in the atmospheric concentration of CO2 and many other environmental parameters like depositional environment, sediment transport pathways, margin type, and post-depositional changes are better constrained. The goals of this dissertation were to 1) assess the influence of these environmental factors on the relationship between carbonate and organic carbon isotope records in a variety of recent depositional environments, including shallow marine platform, slope, and pelagic settings, and 2) to determine whether the assumption that covariance in carbonate and organic carbon isotope values as evidence for a robust record of the carbon isotope values of oceanic DIC is a universally valid assumption. The results of this dissertation suggest that a variety of syn-depositional and post-depositional processes influence the relationship between carbonate and organic carbon isotope records. For example, syn-depositional processes like mixing of isotopically distinct sources produce highly covariant carbonate and organic carbon isotope records in toe of slope and basinal settings adjacent to isolated carbonate platforms (Chapter 3). Post-depositional processes like marine burial diagenesis beneath non-depositional surfaces generate weakly covariant carbonate and organic carbon isotope records from slope sediments adjacent to the Great Barrier Reef (Chapter 4), while repeated episodes of subaerial exposure produce highly covariant carbonate and organic carbon isotope records from the Great Bahama Bank (Chapter 2). Furthermore, analysis of the paired carbonate and organic carbon isotope records from pelagic settings in the Atlantic, Indian, and Pacific Ocean Basins (Chapter 6) show that pelagic depositional environments do not exhibit covariance between carbonate and organic carbon isotope values. In addition to these modern case studies, the analysis of paired carbonate and organic carbon isotope values from four outcrops constituting a transect of shallow-to-deep ramp environments from the Mississippian Madison Limestone in the Western United States demonstrates that large global perturbations can be observed in the carbonate carbon isotope values, but are not captured in the co-occurring organic carbon isotope values (Chapter 5). Finally, the results of these five case studies were synthesized to produce a model of the impact of depositional environment, post-depositional alteration, margin type, and how the influence of these factors on the relationship between carbonate and organic carbon isotope values changed through geologic time (Chapter 6). A significant result of this synthesis is the proposal that icehouse and greenhouse conditions may favor different mechanisms for generating covariance. In concert, these findings have important implications for reconstructing global carbon cycling through geologic time, and may require re-evaluation of some carbon isotope records interpreted to record fundamental biogeochemical changes in the Earth surface system

Staying hydrated in sea water

Reduction of intestinal lumen osmotic pressure by formation of Ca(Mg)CO , "ichthyocarbonate", is essential for osmoregulation by the only vertebrate group, ray-finned fishes, widely capable of hydrating by ingesting seawater. Ichthyocarbonate formation and excretion is under elaborate physiological control and plays an important, yet still poorly defined, role in the oceanic carbon cycle

Revised interpretations of stable C and O patterns in carbonate rocks resulting from meteoric diagenesis

A positive correlation between the δ13C and δ18O values of carbonate rocks is a screening tool widely used to identify the overprint of meteoric diagenesis on the original isotopic composition of a sample. In particular, it has been suggested that systematic change from negative to positive δ13C and δ18O values with increasing depth in the core is an indicator of alteration within the zone of mixing between meteoric and marine waters. In this paper, we propose that such covariance is not generated within the traditionally defined mixing zone, and that positive correlations between δ13C and δ18O values in marine carbonates are not necessarily indicators of meteoric alteration. This new interpretation is based on data collected from the shallow sub-surface of the Bahamas, a region unequivocally influenced by meteoric waters to depths of at least 200m below the current sediment-water interface. The classic interpretation of the diagenetic environments, based on changes in the δ13C and δ18O values, would suggest the maximum penetration of freshwater occurs between 65 and 100m below seafloor. Below these depths, a strong positive covariation between the δ13C and δ18O values exists, and would traditionally be defined as the mixing zone. However, based upon known changes in sea level, the penetration of the freshwater lens extends significantly below this limit. We contend that the zone showing covariance of δ13C and δ18O values is actually altered within the freshwater lens, and not the mixing zone as previously proposed. The co-varying trend in δ13C and δ18O values is the result of diagenetic processes occurring at the interface between vadose and phreatic zones. Significantly greater rates of recrystallization and neomorphism are driven by the increased rates of oxidation of organic matter at this transition with progressively less alteration occurring with increasing depth. As sea level oscillates, the position of this interface moves through the deposit, causing cumulative alteration throughout the section. Hence, we propose that the covariation between δ13C and δ18O values is a consequence of varying degrees of alteration, rather than the result of diagenesis occurring within the zone where marine and freshwater fluids mix. Furthermore, within the pervasively altered vadose zone, there is little correlation between δ13C and δ18O values, and therefore covariation between δ13C and δ18O values is not an unequivocal indicator of meteoric diagenesis

The meaning of dolomite in the deep-sea sedimentary record; implications for global carbon cycle

Although dolomite is not abundant in the Modern, it is a minor and persistent component (500 m) adjacent to several carbonate platforms (Bahamas, Maldives, Great Barrier Reef, and Great Australian Bight) that suggest that higher concentrations of dolomites generally are associated with periods of low rates of deposition. The low rate of deposition is critical as it provides time for the diffusion of Mg (super 2+) and SO (sub 4) (super 2-) from the over lying seawater. Magnesium is necessary for the process of dolomitization while SO (sub 4) (super 2-) is needed to oxidize organic material and thereby raise the alkalinity to promote the dolomitization process. In turn the oxidation of organic material alters the carbon isotope balance and, depending upon the amount of SO (sub 4) (super 2-) supplied, the organic content, and the carbonate content of the sediments, can produce a negative delta (super 13) C value in the bulk sediments. Such changes in the delta (super 13) C values do not represent a global signal and the extent to which they develop is very much dependent upon local sedimentary conditions, such as sediment supply, insoluble content, and slope geometry. A typical dolomite profile beneath a hardground might show a low concentrations immediately at the hardground surface (representing a more open system), but increase with depth, reaching a maximum at a distance dependent upon the time period represented by the hiatus (thus controlling the supply of Mg (super 2+) and SO (sub 4) (super 2-) ) and the amount of organic material available for oxidation (closed system). An interval of reduced sedimentation may show a similar profile, although the maximum concentration of dolomite and influence on the delta (super 13) C values would be reduced. Further support for these processes is provided by additional geochemical tracers such as delta (super 34) S values of the carbonate associated sulfate, delta (super 18) O, delta (super 44) Ca, and delta (super 26) Mg values, and concentrations of strontium in the dolomite

Multi-proxy constraints on the significance of covariant δ13C values in carbonate and organic carbon during the early Mississippian

This study investigates the covariation between carbonate and organic δ13C values in a proximal to distal transect of four outcrops in the Madison Limestone in the Western United States Rockies, combined with δ34S values of carbonate associated sulphate, the concentration of acid-insoluble material and measurements of total organic carbon. These new geochemical datasets not only allow for an evaluation of carbon isotope covariance during one of the largest perturbations to the global carbon cycle over the past 550 Myr, but also constrain the cause of the excursion in carbonate δ13C values. The results support the hypothesis that a period of anoxia did not play a role in generating the positive carbonate δ13C values, but rather favour interpretations by previous workers that the proliferation of land plants destabilized the Carboniferous carbon cycle, setting the stage for a significant change in the carbonate δ13C values of contemporaneous marine carbonates. These results also demonstrate that one of the largest perturbations to the global carbon cycle did not produce synchronous variations in carbonate and organic δ13C values, emphasizing the importance of local depositional controls on carbon isotope covariance in the geological record in both modern and ancient environments

SIMULATING METEORIC AND MIXING ZONE CARBONATE DIAGENESIS WITH A TWO-DIMENSIONAL REACTIVE TRANSPORT MODEL

Meteoric and mixing-zone diagenesis can dramatically alter the geochemical signatures of shallow marine carbonates. Most preserved pre-Cretaceous carbonates were deposited in shallow marine environments and thus may have been susceptible to meteoric and mixing-zone diagenesis. However, a quantitative understanding of how the geochemical composition of carbonates changes during diagenesis still requires further development. Here, we present a new two-dimensional (2D) reactive transport model coupled with a 2D coastal hydrology model to simulate carbonate diagenesis and provide insights into its impact on the isotopic and elemental compositions of carbonates in the geological record. Using this model, we have simulated the stratigraphic trends and relationship between isotopic records (for example, delta C-13 and delta O-18 values) observed in modern (Recent-Miocene) sections where the impact of meteoric diagenesis has been dearly characterized. Our model can also reproduce anomalous Neoproterozoic carbonate geochemical profiles where the effects of meteoric diagenesis have been debated. Further, our model indicates that linear carbonate GO isotope co-variations can either be generated in the mixing zone between freshwater and marine pore waters, or in the freshwater phreatic zone with a downward decrease in recrystallization (with no net carbonate dissolution or precipitation) rate. In addition, numerous processes were observed to decouple delta O-18(carb) values from other isotopic and elemental signatures during carbonate diagenesis, indicating that a lack of linear correlation between delta O-18(carb) values and other geochemical variables does not necessarily suggest limited meteoric alteration. Sensitivity analyses show that the steady-state timescale is controlled by compositional differences between fluid endmembers, the calcite-water element distribution coefficient, the recrystallization rate, porosity, and the groundwater discharge rate. Given that reactive transport models have proven to be powerful theoretical tools in many disciplines of Earth sciences, our hope is that this model will promote a more quantitative understanding of meteoric and mixing zone diagenesis of marine carbonates

Reconstructing paleoceanographic conditions during the middle Ediacaran: Evidence from giant ooids in South China

The Ediacaran was a period of intense environmental change in Earth history. Transient pulses of oxygenation and shifts in ocean chemistry have been interpreted from a range of geochemical proxies, including rare earth elements and yttrium (REY), but it is still a challenge to reconstruct paleoceanographic conditions due to the scarcity of reliable recorder. In this study, we explore the possibility that a newly-discovered deposit of giant ooids in the Nanshanping outcrop in South China contains a relatively intact record of the REE composition of Ediacaran seawater, providing an important window into the redox state of the surface ocean at this time. These ooids are relatively large in size (similar to 1-6 mm), and exhibit densely banded laminae. A regional chemostratigraphic correlation shows that the giant-ooid-bearing succession in South China is in the upper part of the second Ediacaran positive carbon isotopic shift (EP2), and thus its occurrence was not associated with the Shuram Excursion. Additionally, the occurrences of giant-ooid-bearing oolites in different blocks were not synchronous during the middle Ediacaran. In-situ laser ablation analysis of giant-ooid cortices reveals that the shale-normalized REY patterns are similar to those of modern seawater, including depleted light REEs, superchondritic Y/Ho ratios, and positive La anomalies. Yet, no Ce anomalies (1.0 +/- 0.15) in numerous ooid cortices seems to indicate a suboxidized condition of surface ocean in South China at that time. The petrographic and geochemical evidence support the weak diagenetic alteration of the pure giant ooids during burial. They formed in open-ocean settings with energetic hydrodynamic environments and were affected by early dolomitization that might preserve mostly original petrographic fabrics and REY signatures from further diagenetic overprinting. The characteristics of Ce anomalies and REY compositions acquired from the giant ooids indicate that the surface ocean was suboxidized in South China during the late stage of EP2, implying the very low atmospheric O-2 level at that time. Our results suggest that giant ooids may be a reliable analytical target that provide a snapshot into palaeoceanographic conditions during this crucial period of dynamic changes in early Earth history

Hidden in plain sight; ubiquitous authigenic carbonate in a modern slope setting

The geological record is periodically punctuated by large negative changes in the carbon isotope composition of marine carbonate sediments. Traditional interpretation of these types of excursions invokes an origin driven by the exogenic carbon cycle. Thus, the evolution of new biogeochemical metabolisms, atmospheric oxygenation, and weathering processes have previously been inextricably linked to the carbon isotope composition of marine carbonates. Recent studies have suggested that these large negative changes can be generated by a process unrelated to global carbon cycling: the formation of authigenic carbonate. The diagenetic origin of authigenic carbonates decouples their carbon isotopic composition from that of the surface oceans and atmosphere, thereby adding a new dimension of complexity to global carbon cycle models. Given the newfound importance of authigenic carbonate as a carbon sink, constraining the drivers, rate of formation, and amount of authigenic precipitation is crucial. Here we present evidence of authigenic carbonate formed in oxygenated bottom waters in sediments collected during ODP Leg 133 from the mixed carbonate-siliciclastic slope adjacent to the Great Barrier Reef in Australia. Our results produce new constraints on the timing, quantity, and drivers for authigenic carbonate formation, and identify the role of reduced sedimentation rate which provides an essentially unlimited dissolved sulfate supply for authigenic precipitation. Under such conditions, modeled precipitation of authigenic carbonate ranging from 5-15% can reproduce the observed -4 ppm change in carbon isotope values during the short ( approximately 200 kyr) period of reduced sedimentation without obvious sedimentological evidence of diagenesis. These results provide a mechanism to explain asynchronous, punctuated negative shifts in carbon isotope values of marine carbonates unrelated to changes in global carbon cycling or periods of bottom water anoxia