Posts Tagged 'paleo'

Incorporation of minor and trace elements into cultured brachiopods: implications for proxy application with new insights from a biomineralisation model

Brachiopods present a key fossil group for Phanerozoic palaeo-environmental and palaeo-oceanographical reconstructions, owing to their good preservation and abundance in the geological record. Yet to date, hardly any geochemical proxies have been calibrated in cultured brachiopods and only little is known on the mechanisms that control the incorporation of various key elements into brachiopod calcite. To evaluate the feasibility and robustness of multiple Element/Ca ratios as proxies in brachiopods, specifically Li/Ca, B/Ca, Na/Ca, Mg/Ca, Sr/Ca, Ba/Ca, as well as Li/Mg, we cultured Magellania venosa, Terebratella dorsata and Pajaudina atlantica under controlled experimental settings over a period of more than two years with closely monitored ambient conditions, carbonate system parameters and elemental composition of the culture medium. The experimental setup comprised of two control aquariums (pH0 = 8.0 and 8.15, T = 10 °C) and treatments where pCO2 − pH (pH1 = 7.6 and pH2 = 7.35), temperature (T = 16 °C) and chemical composition of the culture medium were manipulated. Our results indicate that the incorporation of Li and Mg is strongly influenced by temperature, growth effects as well as carbonate chemistry, complicating the use of Li/Ca, Mg/Ca and Li/Mg ratios as straightforward reliable proxies. Boron partitioning varied greatly between the treatments, however without a clear link to carbonate system parameters or other environmental factors. The partitioning of both Ba and Na varied between individuals, but was not systematically affected by changes in the ambient conditions. We highlight Sr as a potential proxy for DIC, based on a positive trend between Sr partitioning and carbonate chemistry in the culture medium. To explain the observed dependency and provide a quantitative framework for exploring elemental variations, we devise the first biomineralisation model for brachiopods, which results in a close agreement between modelled and measured Sr distribution coefficients. We propose that in order to sustain shell growth under increased DIC, a decreased influx of Ca2+ to the calcifying fluid is necessary, driving the preferential substitution of Sr2+ for Ca2+ in the crystal lattice. Finally, we conducted micro-computed tomography analyses of the shells grown in the different experimental treatments. We present pore space – punctae – content quantification that indicates that shells built under increased environmental stress, and in particular elevated temperature, contain relatively more pore space than calcite, suggesting this parameter as a potential novel proxy for physiological stress and even environmental conditions.

Continue reading ‘Incorporation of minor and trace elements into cultured brachiopods: implications for proxy application with new insights from a biomineralisation model’

Palaeoclimate ocean conditions shaped the evolution of corals and their skeletons through deep time

Identifying how past environmental conditions shaped the evolution of corals and their skeletal traits provides a framework for predicting their persistence and that of their non-calcifying relatives under impending global warming and ocean acidification. Here we show that ocean geochemistry, particularly aragonite–calcite seas, drives patterns of morphological evolution in anthozoans (corals, sea anemones) by examining skeletal traits in the context of a robust, time-calibrated phylogeny. The lability of skeletal composition among octocorals suggests a greater ability to adapt to changes in ocean chemistry compared with the homogeneity of the aragonitic skeleton of scleractinian corals. Pulses of diversification in anthozoans follow mass extinctions and reef crises, with sea anemones and proteinaceous corals filling empty niches as tropical reef builders went extinct. Changing environmental conditions will likely diminish aragonitic reef-building scleractinians, but the evolutionary history of the Anthozoa suggests other groups will persist and diversify in their wake.

Continue reading ‘Palaeoclimate ocean conditions shaped the evolution of corals and their skeletons through deep time’

Controls on the spatio-temporal distribution of microbialite crusts on the Great Barrier Reef over the past 30,000 years

Highlights

  • Comprehensive dataset of reefal microbial crusts over the past 30,000 years.
  • Modern 3D analysis to assess heterogeneity of microbialites in reef frameworks.
  • Radiocarbon ages show microbialite development coeval with and postdating framework.
  • Microbialite thickness correlates with changes in carbonate saturation level and pH.

Abstract

Calcification of microbial mats adds significant amounts of calcium carbonate to primary coral reef structures that stabilizes and binds reef frameworks. Previous studies have shown that the distribution and thicknesses of late Quaternary microbial crusts have responded to changes in environmental parameters such as seawater pH, carbonate saturation state, and sediment and nutrient fluxes. However, these studies are few and limited in their spatio-temporal coverage. In this study, we used 3D and 2D examination techniques to investigate the spatio-temporal distribution of microbial crusts and their responses to environmental changes in Integrated Ocean Drilling Program (IODP) Expedition 325 (Great Barrier Reef Environmental Changes) fossil reef cores that span 30 to 10 ka at two locations on the GBR reef margin. Our GBR microbialite record was then combined with a meta-analysis of 17 other reef records to assess global scale changes in microbialite development (i.e., presence/absence, thickness) over the same period. The 3D results were compared with 2D surface area measurements to assess the accuracy of 2D methodology. The 2D technique represents an efficient and accurate proxy for the 3D volume of reef framework components within the bounds of uncertainty (average: 9.45 ± 4.5%). We found that deep water reef frameworks were most suitable for abundant microbial crust development. Consistent with a previous Exp. 325 study (Braga et al., 2019), we also found that crust ages were broadly coeval with coralgal communities in both shallow water and fore-reef settings. However, in some shallow water settings they also occur as the last reef framework binding stage, hundreds of years after the demise of coralgal communities. Lastly, comparisons of crust thickness with changes in environmental conditions between 30 and 10 ka, show a temporal correlation with variations in partial pressure of CO2 (pCO2), calcite saturation state (Ωcalcite), and pH of seawater, particularly during the past ~15 kyr, indicating that these environmental factors likely played a major role in microbialite crust development in the GBR. This supports the view that microbialite crust development can be used as an indicator of ocean acidification.

Continue reading ‘Controls on the spatio-temporal distribution of microbialite crusts on the Great Barrier Reef over the past 30,000 years’

Ocean acidification during the early Toarcian extinction event: evidence from boron isotopes in brachiopods

The loss of carbonate production during the Toarcian Oceanic Anoxic Event (T-OAE, ca. 183 Ma) is hypothesized to have been at least partly triggered by ocean acidification linked to magmatism from the Karoo-Ferrar large igneous province (southern Africa and Antarctica). However, the dynamics of acidification have never been directly quantified across the T-OAE. Here, we present the first record of temporal evolution of seawater pH spanning the late Pliensbachian and early Toarcian from the Lusitanian Basin (Portugal) reconstructed on the basis of boron isotopic composition (δ11B) of brachiopod shells. δ11B declines by ~1‰ across the Pliensbachian-Toarcian boundary (Pl-To) and attains the lowest values (~12.5‰) just prior to and within the T-OAE, followed by fluctuations and a moderately increasing trend afterwards. The decline in δ11B coincides with decreasing bulk CaCO3 content, in parallel with the two-phase decline in carbonate production observed at global scales and with changes in pCO2 derived from stomatal indices. Seawater pH had declined significantly already prior to the T-OAE, probably due to the repeated emissions of volcanogenic CO2. During the earliest phase of the T-OAE, pH increased for a short period, likely due to intensified continental weathering and organic carbon burial, resulting in atmospheric CO2 drawdown. Subsequently, pH dropped again, reaching the minimum in the middle of the T-OAE. The early Toarcian marine extinction and carbonate collapse were thus driven, in part, by ocean acidification, similar to other Phanerozoic events caused by major CO2 emissions and warming.

Continue reading ‘Ocean acidification during the early Toarcian extinction event: evidence from boron isotopes in brachiopods’

Geochemical reconstructions of Southern Ocean pH and temperature over the last glacial cycle

The Southern Ocean is widely thought to play an important role in atmospheric CO₂ change over glacial-interglacial cycles. It has been suggested that as the region that ventilates the majority of the world’s carbon-rich deep waters today, reduced exchange between deep waters and the atmosphere in the Southern Ocean acted to draw down CO₂ over glacial timescales. However, direct evidence of the Southern Ocean’s role in glacial CO₂ drawdown has been lacking thus far. Here I apply the boron-isotope pH-proxy to foraminifera from the Antarctic Zone sediment core PS1506 over the last glacial cycle. The low boron concentrations in these polar foraminifera makes these samples particularly sensitive to boron blank and so a close examination of the sources of blank, and an assessment of the precision of blank measurements, has been made. The ratios of trace elements to calcium in foraminiferal shells are widely applied as proxies for palaeoenvironmental parameters such as temperature. As Southern Ocean carbonate sediments are particularly prone to dissolution, which can affect trace element concentrations, an assessment of dissolution has been made. Firstly, dissolution experiments were conducted to constrain the impact of dissolution in a controlled setting, and secondly, shell mass and trace elements were evaluated for the downcore record. Imaging reveals similar etching textures in both experimentally dissolved samples and deglacial intervals, when shell mass is also low and several trace elements exhibit an excursion to lower values. Boron isotope data for PS1506 show that during the penultimate interglacial, surface water pH was low. At the onset of atmospheric CO₂ drawdown, pH increased, indicating low CO₂ surface waters. This is consistent with the signature predicted for a more stratified Southern Ocean, and is evidence that stratification in the Antarctic Zone acted to contribute to CO₂ drawdown early in the transition to a glacial state.

Continue reading ‘Geochemical reconstructions of Southern Ocean pH and temperature over the last glacial cycle’

Ocean acidification and photic‐zone anoxia at the Toarcian Oceanic Anoxic Event: insights from the Adriatic Carbonate Platform

Severe global climate change led to the deterioration of environmental conditions in the oceans during the Toarcian Stage of the Jurassic. Carbonate platforms of the Western Tethys Ocean exposed in Alpine Tethyan mountain ranges today offer insight into this period of environmental upheaval. In addition to informing understanding of climate change in deep time, the effect of ancient carbon cycle perturbations on carbonate platforms have important implications for anthropogenic climate change; the patterns of early Toarcian environmental deterioration are similar to those occurring in modern oceans. This study focuses on the record of the early Toarcian Oceanic Anoxic Event (ca 183.1 Ma) in outcrops of the north‐west Adriatic Carbonate Platform in Slovenia. Amidst environmental deterioration, the north‐west Adriatic Platform abruptly transitioned from a healthy, shallow‐water environment with diverse metazoan ecosystems to a partially drowned setting with low diversity biota and diminished sedimentation. An organic carbon‐isotope excursion of ‐2.2‰ reflects the massive injection of CO2 into the ocean‐atmosphere system and marks the stratigraphic position of the Toarcian Oceanic Anoxic Event. A prominent dissolution horizon and suppressed carbonate deposition on the platform are interpreted to reflect transient shoaling of the carbonate compensation depth to unprecedentedly shallow levels as the dramatic influx of CO2 overwhelmed the ocean’s buffering capacity, causing ocean acidification. Trace metal geochemistry and palaeoecology highlight water column deoxygenation, including the development of photic‐zone anoxia, preceding and during the Toarcian Oceanic Anoxic Event. Ocean acidification and reduced oxygen levels likely had a profoundly negative effect on carbonate‐producing biota and growth of the Adriatic Platform. These effects are consistent with the approximate doubling of the concentration of CO2 in the ocean‐atmosphere system from pre‐Toarcian Oceanic Anoxic Event levels, which has previously been linked to a volcanic triggering mechanism. Mercury enrichments discovered in this study support a temporal and genetic link between volcanism, the Toarcian Oceanic Anoxic Event and the carbonate crisis.

Continue reading ‘Ocean acidification and photic‐zone anoxia at the Toarcian Oceanic Anoxic Event: insights from the Adriatic Carbonate Platform’

Late Holocene climate variability and coastal change of the Yucatan Peninsula, Mexico

The following dissertation contains three studies that use sediment cores to reconstruct past changes in the climate and environment of a tropical lagoon system. These studies provide insight into past droughts and coastal change during geologically recent climate variability and sea-level rise by investigating relationships between geochemical and biological parameters sensitive to different processes occurring on the coast of the Yucatan Peninsula, Mexico.

Chapter one is a foraminiferal fossil record reconstruction of the Celestun Lagoon environment, assessing ecologic response to a change in lagoon salinity and vegetation over the Late Holocene (5,300 years to present). The record and modern environment suggests foraminiferal community composition changes predictably with salinity, but lagoon salinity decreased primarily from restriction of seawater input to the lagoon, and hence reduced mixing between groundwater and seawater, rather than climate-induced increase of groundwater discharge, though climate is a secondary control. The cause of reduced mixing appears to be accumulation of barrier islands and sand spits that progressively isolated the northern lagoon, reducing mixing between groundwater discharge and seawater and shifting the environment from an open marine coast to estuarine lagoon. The transition was accompanied by expansion of the mangrove forest fringing the coastline. Superimposed on this trend, excursions of foraminifera taxa signify higher salinity coinciding with regionally dry periods and indicate that climate is a second-order control on lagoon mean salinity.

Chapter two is a more detailed paleosalinity reconstruction where relations between modern lagoon salinity and both trace metals and isotopes in foraminiferal tests are applied to samples from cores collected along a transect from the northern to southern lagoon. The benthic species Ammonia parkinsoniana is used due to its abundance throughout the lagoon, and paleosalinity tracers recorded in A. parkinsoniana calcite tests are the elemental ratios Sr/Ca, and Ba/Ca and isotopes δ18O and 87Sr/86Sr. Ba/Ca ratios exhibit the highest correlation with salinity while δ18O and 87Sr/86Sr indicate two types of groundwater discharge to the lagoon—a fresh and a brackish source. A mixing model constructed from δ18O and 87Sr/86Sr show that long-term decrease in salinity was due to increased proportions of the brackish groundwater endmember—consistent with the Chapter 1—and decreases in the freshwater endmember coincide with major dry periods in the Yucatan recorded in other paleoclimate archives of the region. Furthermore, sedimentation rates increase briefly at 3,400 and 2,000 years ago, time periods characterized by large-scale reorganization of atmospheric and oceanic currents in the North Atlantic with atmospheric teleconnections to tropical climate. These increases in accumulation rate are interpreted as periods of rapid barrier island accumulation as trade winds and the Loop Current weaken in the Gulf of Mexico and deposit sediments during longshore drift. Chapter two suggests that atmospheric patterns resulting in drought in the Yucatan Peninsula also result in rapid sedimentation and apparent decrease in salinity in coastal lagoons, thus demonstrating the value of a multi-proxy approach in reconstructing paleoenvironmental history in dynamic coastal environments.

Chapter three contributes data and a new hypothesis to the growing body of literature on the boron isotope system. The boron isotope ratio 11B/10B records pH of ambient water in the carbonate shells, proving to be a powerful tool in reconstructing past ocean acidification and atmospheric carbon dioxide concentrations. However, δ11B has not previously been used as a proxy for low-pH spring water discharge. In Celestun Lagoon, boron measurements in A. parkinsoniana are characterized by high variability both in surface sediments along the lagoon and in downcore samples and exhibit weak but significant relationships with the paleosalinity proxies 87Sr/86Sr and Ba/Ca and with the vegetation proxy δ13C. Lower pH caused by respiration of organic matter, recorded in δ13C of calcite, appears to contribute to δ11B variability, yet mean δ11B values of calcite reflect calculated δ11B values of borate based on present understanding of boron systematics, thus indicating that spring discharge exerts a first-order control on lagoon pH and δ11B recorded in foraminifera. This finding is of particular interest to the deep-time paleocommunity because prior to the evolution of foraminifera, and because deep sea sediments older than 180 million years are rare, many calcareous fossils available for δ11B analysis thrived in shallow marine habitats. As efforts continue to find deep-time analogs to modern ocean acidification, low-pH groundwater discharge in coastal zones may complicate interpretations of δ11B results but may be addressed by a rigorous multi-proxy approach.

This dissertation provides a record of coastal and climate change during recent periods of climate variability and sea-level rise over the last 5,000 years to provide context for current climate change in the tropics and an understanding of drivers of variability in the past and the future at low latitude sites.

Continue reading ‘Late Holocene climate variability and coastal change of the Yucatan Peninsula, Mexico’

Seawater pH reconstruction using boron isotopes in multiple planktonic foraminifera species with different depth habitats and their potential to constrain pH and pCO2 gradients (update)

Boron isotope systematics of planktonic foraminifera from core-top sediments and culture experiments have been studied to investigate the sensitivity of δ11B of calcite tests to seawater pH. However, our knowledge of the relationship between δ11B and pH remains incomplete for many taxa. Thus, to expand the potential scope of application of this proxy, we report δ11B data for seven different species of planktonic foraminifera from sediment core tops. We utilize a method for the measurement of small samples of foraminifera and calculate the δ11B-calcite sensitivity to pH for Globigerinoides ruber, Trilobus sacculifer (sacc or without sacc), Orbulina universa, Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, Globorotalia menardii, and Globorotalia tumida, including for unstudied core tops and species. These taxa have diverse ecological preferences and are from sites that span a range of oceanographic regimes, including some that are in regions of air–sea equilibrium and others that are out of equilibrium with the atmosphere. The sensitivity of δ11Bcarbonate to δ11Bborate (e.g., Δδ11Bcarbonate∕Δδ11Bborate) in core tops is consistent with previous studies for T. sacculifer and G. ruber and close to unity for N. dutertrei, O. universa, and combined deep-dwelling species. Deep-dwelling species closely follow the core-top calibration for O. universa, which is attributed to respiration-driven microenvironments likely caused by light limitation and/or symbiont–host interactions. Our data support the premise that utilizing boron isotope measurements of multiple species within a sediment core can be utilized to constrain vertical profiles of pH and pCO2 at sites spanning different oceanic regimes, thereby constraining changes in vertical pH gradients and yielding insights into the past behavior of the oceanic carbon pumps.

Continue reading ‘Seawater pH reconstruction using boron isotopes in multiple planktonic foraminifera species with different depth habitats and their potential to constrain pH and pCO2 gradients (update)’

The pH dependency of the boron isotopic composition of diatom opal (Thalassiosira weissflogii) (update)

The high-latitude oceans are key areas of carbon and heat exchange between the atmosphere and the ocean. As such, they are a focus of both modern oceanographic and palaeoclimate research. However, most palaeoclimate proxies that could provide a long-term perspective are based on calcareous organisms, such as foraminifera, that are scarce or entirely absent in deep-sea sediments south of 50 S in the Southern Ocean and north of 40 N in the North Pacific. As a result, proxies need to be developed for the opal-based organisms (e.g. diatoms) found at these high latitudes, which dominate the biogenic sediments recovered from these regions. Here we present a method for the analysis of the boron (B) content and isotopic composition (δ11B) of diatom opal. We apply it for the first time to evaluate the relationship between seawater pH, δ11B and B concentration ([B]) in the frustules of the diatom Thalassiosira weissflogii, cultured across a range of carbon dioxide partial pressure (pCO2) and pH values. In agreement with existing data, we find that the [B] of the cultured diatom frustules increases with increasing pH (Mejía et al., 2013). δ11B shows a relatively well defined negative trend with increasing pH, completely distinct from any other biomineral previously measured. This relationship not only has implications for the magnitude of the isotopic fractionation that occurs during boron incorporation into opal, but also allows us to explore the potential of the boron-based proxies for palaeo-pH and palaeo-CO2 reconstruction in high-latitude marine sediments that have, up until now, eluded study due to the lack of suitable carbonate material.

Continue reading ‘The pH dependency of the boron isotopic composition of diatom opal (Thalassiosira weissflogii) (update)’

Towards an understanding of the Ca isotopic signal related to ocean acidification and alkalinity overshoots in the rock record

In this contribution, we explore the idea that the Ca isotope proxy has utility as an indicator of carbonate authigenesis (i.e., post-depositional precipitation of CaCO3 within the sedimentary package). Given the strong contrast in isotopic fractionation factor between the formational and diagenetic environments, Ca isotopes have the potential to fingerprint carbonate authigenesis when it occurs close to the seawater-sediment interface. We demonstrate that Ca isotopes are particularly applicable to exploring ocean acidification events, and potentially ocean anoxic events, and focus our attention on ocean acidification related to the Paleocene-Eocene Thermal Maximum (PETM). We present three scenarios that vary in magnitude and duration of carbon fluxes simulated using an Earth System model of intermediate complexity (cGENIE) and use the cGENIE output to constrain the upper boundary conditions of 1-D reactive transport models of authigenesis and recrystallization in the sedimentary section. Along with simple mixing calculations, the models inform our exploration of the hypothesis that authigenic carbonate induced by a saturation state overshoot during the PETM explains recently published Ca isotope records, and perhaps bulk carbonate records over Ocean Anoxic Event (OAE) 2. Our simulations suggest that fractionation factor variability does not explain the PETM δ44Ca records, and we propose a δ44Ca-CaCO3 space framework to assist with the elucidation of authigenic additions over time scales that are short relative to the residence time of Ca in the ocean (~1 Ma). Ultimately, we find that the ‘authigenic zone’ generated in the sedimentary column may be influenced by alkalinity overshoots or redox state; the CaCO3 produced in this zone can overprint temporal signals with depth-dependent signals that reflect lithology and sedimentation rate and need not be spatially uniform, even when driven by a global event. Ultimately, we demonstrate the utility of Ca isotopes for exploring short time scale climatic events and a quantitative framework to guide interpretations.

Continue reading ‘Towards an understanding of the Ca isotopic signal related to ocean acidification and alkalinity overshoots in the rock record’

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Ocean acidification in the IPCC AR5 WG II

OUP book