Publications by authors named "Andy Ridgwell"

30 Publications

  • Page 1 of 1

Temperature controls carbon cycling and biological evolution in the ocean twilight zone.

Science 2021 03;371(6534):1148-1152

School of Earth and Environmental Sciences, Cardiff University, Cardiff, UK.

Theory suggests that the ocean's biological carbon pump, the process by which organic matter is produced at the surface and transferred to the deep ocean, is sensitive to temperature because temperature controls photosynthesis and respiration rates. We applied a combined data-modeling approach to investigate carbon and nutrient recycling rates across the world ocean over the past 15 million years of global cooling. We found that the efficiency of the biological carbon pump increased with ocean cooling as the result of a temperature-dependent reduction in the rate of remineralization (degradation) of sinking organic matter. Increased food delivery at depth prompted the development of new deep-water niches, triggering deep plankton evolution and the expansion of the mesopelagic "twilight zone" ecosystem.
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http://dx.doi.org/10.1126/science.abb6643DOI Listing
March 2021

Orographic evolution of northern Tibet shaped vegetation and plant diversity in eastern Asia.

Sci Adv 2021 Jan 27;7(5). Epub 2021 Jan 27.

CAS Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla 666303, China.

The growth of the Tibetan Plateau throughout the past 66 million years has profoundly affected the Asian climate, but how this unparalleled orogenesis might have driven vegetation and plant diversity changes in eastern Asia is poorly understood. We approach this question by integrating modeling results and fossil data. We show that growth of north and northeastern Tibet affects vegetation and, crucially, plant diversity in eastern Asia by altering the monsoon system. This northern Tibetan orographic change induces a precipitation increase, especially in the dry (winter) season, resulting in a transition from deciduous broadleaf vegetation to evergreen broadleaf vegetation and plant diversity increases across southeastern Asia. Further quantifying the complexity of Tibetan orographic change is critical for understanding the finer details of Asian vegetation and plant diversity evolution.
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http://dx.doi.org/10.1126/sciadv.abc7741DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7840128PMC
January 2021

Bistability in the redox chemistry of sediments and oceans.

Proc Natl Acad Sci U S A 2020 12 14;117(52):33043-33050. Epub 2020 Dec 14.

Department of Biology, University of Antwerp, 2160 Wilrijk, Belgium;

For most of Earth's history, the ocean's interior was pervasively anoxic and showed occasional shifts in ocean redox chemistry between iron-buffered and sulfide-buffered states. These redox transitions are most often explained by large changes in external inputs, such as a strongly altered delivery of iron and sulfate to the ocean, or major shifts in marine productivity. Here, we propose that redox shifts can also arise from small perturbations that are amplified by nonlinear positive feedbacks within the internal iron and sulfur cycling of the ocean. Combining observational evidence with biogeochemical modeling, we show that both sedimentary and aquatic systems display intrinsic iron-sulfur bistability, which is tightly linked to the formation of reduced iron-sulfide minerals. The possibility of tipping points in the redox state of sediments and oceans, which allow large and nonreversible geochemical shifts to arise from relatively small changes in organic carbon input, has important implications for the interpretation of the geological rock record and the causes and consequences of major evolutionary transitions in the history of Earth's biosphere.
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http://dx.doi.org/10.1073/pnas.2008235117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7776822PMC
December 2020

Past climates inform our future.

Science 2020 11;370(6517)

Department of Oceanography, Texas A&M University, College Station, TX, USA.

As the world warms, there is a profound need to improve projections of climate change. Although the latest Earth system models offer an unprecedented number of features, fundamental uncertainties continue to cloud our view of the future. Past climates provide the only opportunity to observe how the Earth system responds to high carbon dioxide, underlining a fundamental role for paleoclimatology in constraining future climate change. Here, we review the relevancy of paleoclimate information for climate prediction and discuss the prospects for emerging methodologies to further insights gained from past climates. Advances in proxy methods and interpretations pave the way for the use of past climates for model evaluation-a practice that we argue should be widely adopted.
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http://dx.doi.org/10.1126/science.aay3701DOI Listing
November 2020

Algal plankton turn to hunting to survive and recover from end-Cretaceous impact darkness.

Sci Adv 2020 Oct 30;6(44). Epub 2020 Oct 30.

Earth and Planetary Sciences, University of California at Riverside, Riverside, CA 92521, USA.

The end-Cretaceous bolide impact triggered the devastation of marine ecosystems. However, the specific kill mechanism(s) are still debated, and how primary production subsequently recovered remains elusive. We used marine plankton microfossils and eco-evolutionary modeling to determine strategies for survival and recovery, finding that widespread phagotrophy (prey ingestion) was fundamental to plankton surviving the impact and also for the subsequent reestablishment of primary production. Ecological selectivity points to extreme post-impact light inhibition as the principal kill mechanism, with the marine food chain temporarily reset to a bacteria-dominated state. Subsequently, in a sunlit ocean inhabited by only rare survivor grazers but abundant small prey, it was mixotrophic nutrition (autotrophy and heterotrophy) and increasing cell sizes that enabled the eventual reestablishment of marine food webs some 2 million years later.
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http://dx.doi.org/10.1126/sciadv.abc9123DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7608818PMC
October 2020

The impact of marine nutrient abundance on early eukaryotic ecosystems.

Geobiology 2020 03 17;18(2):139-151. Epub 2020 Feb 17.

NASA Astrobiology Institute, Alternative Earths Team, Riverside, California.

The rise of eukaryotes to ecological prominence represents one of the most dramatic shifts in the history of Earth's biosphere. However, there is an enigmatic temporal lag between the emergence of eukaryotic organisms in the fossil record and their much later ecological expansion. In parallel, there is evidence for a secular increase in the availability of the key macronutrient phosphorus (P) in Earth's oceans. Here, we use an Earth system model equipped with a size-structured marine ecosystem to explore relationships between plankton size, trophic complexity, and the availability of marine nutrients. We find a strong dependence of planktonic ecosystem structure on ocean nutrient abundance, with a larger ocean nutrient inventory leading to greater overall biomass, broader size spectra, and increasing abundance of large Zooplankton. If existing estimates of Proterozoic marine nutrient levels are correct, our results suggest that increases in the ecological impact of eukaryotic algae and trophic complexity in eukaryotic ecosystems were directly linked to restructuring of the global P cycle associated with the protracted rise of surface oxygen levels. Our results thus suggest an indirect but potentially important mechanism by which ocean oxygenation may have acted to shape marine ecological function during late Proterozoic time.
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http://dx.doi.org/10.1111/gbi.12384DOI Listing
March 2020

Fundamentally different global marine nitrogen cycling in response to severe ocean deoxygenation.

Proc Natl Acad Sci U S A 2019 12 25;116(50):24979-24984. Epub 2019 Nov 25.

School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, United Kingdom.

The present-day marine nitrogen (N) cycle is strongly regulated by biology. Deficiencies in the availability of fixed and readily bioavailable nitrogen relative to phosphate (P) in the surface ocean are largely corrected by the activity of diazotrophs. This feedback system, termed the "nitrostat," is thought to have provided close regulation of fixed-N speciation and inventory relative to P since the Proterozoic. In contrast, during intervals of intense deoxygenation such as Cretaceous ocean anoxic event (OAE) 2, a few regional sedimentary δN records hint at the existence of a different mode of marine N cycling in which ammonium plays a major role in regulating export production. However, the global-scale dynamics during this time remain unknown. Here, using an Earth System model and taking the example of OAE 2, we provide insights into the global marine nitrogen cycle under severe ocean deoxygenation. Specifically, we find that the ocean can exhibit fundamental transitions in the species of nitrogen dominating the fixed-N inventory--from nitrate (NO) to ammonium (NH)--and that as this transition occurs, the inventory can partially collapse relative to P due to progressive spatial decoupling between the loci of NH oxidation, NO reduction, and nitrogen fixation. This finding is relatively independent of the specific state of ocean circulation and is consistent with nitrogen isotope and redox proxy data. The substantive reduction in the ocean fixed-N inventory at an intermediate state of deoxygenation may represent a biogeochemical vulnerability with potential implications for past and future (warmer) oceans.
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http://dx.doi.org/10.1073/pnas.1905553116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6911173PMC
December 2019

Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact.

Proc Natl Acad Sci U S A 2019 11 21;116(45):22500-22504. Epub 2019 Oct 21.

Department of Geology & Geophysics, Yale University, New Haven, CT 06520;

Mass extinction at the Cretaceous-Paleogene (K-Pg) boundary coincides with the Chicxulub bolide impact and also falls within the broader time frame of Deccan trap emplacement. Critically, though, empirical evidence as to how either of these factors could have driven observed extinction patterns and carbon cycle perturbations is still lacking. Here, using boron isotopes in foraminifera, we document a geologically rapid surface-ocean pH drop following the Chicxulub impact, supporting impact-induced ocean acidification as a mechanism for ecological collapse in the marine realm. Subsequently, surface water pH rebounded sharply with the extinction of marine calcifiers and the associated imbalance in the global carbon cycle. Our reconstructed water-column pH gradients, combined with Earth system modeling, indicate that a partial ∼50% reduction in global marine primary productivity is sufficient to explain observed marine carbon isotope patterns at the K-Pg, due to the underlying action of the solubility pump. While primary productivity recovered within a few tens of thousands of years, inefficiency in carbon export to the deep sea lasted much longer. This phased recovery scenario reconciles competing hypotheses previously put forward to explain the K-Pg carbon isotope records, and explains both spatially variable patterns of change in marine productivity across the event and a lack of extinction at the deep sea floor. In sum, we provide insights into the drivers of the last mass extinction, the recovery of marine carbon cycling in a postextinction world, and the way in which marine life imprints its isotopic signal onto the geological record.
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http://dx.doi.org/10.1073/pnas.1905989116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6842625PMC
November 2019

Diversity decoupled from ecosystem function and resilience during mass extinction recovery.

Nature 2019 10 25;574(7777):242-245. Epub 2019 Sep 25.

Department of Earth Sciences, University of California at Riverside, Riverside, CA, USA.

The Chicxulub bolide impact 66 million years ago drove the near-instantaneous collapse of ocean ecosystems. The devastating loss of diversity at the base of ocean food webs probably triggered cascading extinctions across all trophic levels and caused severe disruption of the biogeochemical functions of the ocean, and especially disrupted the cycling of carbon between the surface and deep sea. The absence of sufficiently detailed biotic data that span the post-extinction interval has limited our understanding of how ecosystem resilience and biochemical function was restored; estimates of ecosystem 'recovery' vary from less than 100 years to 10 million years. Here, using a 13-million-year-long nannoplankton time series, we show that post-extinction communities exhibited 1.8 million years of exceptional volatility before a more stable equilibrium-state community emerged that displayed hallmarks of resilience. The transition to this new equilibrium-state community with a broader spectrum of cell sizes coincides with indicators of carbon-cycle restoration and a fully functioning biological pump. These findings suggest a fundamental link between ecosystem recovery and biogeochemical cycling over timescales that are longer than those suggested by proxies of export production, but far shorter than the return of taxonomic richness. The fact that species richness remained low as both community stability and biological pump efficiency re-emerged suggests that ecological functions rather than the number of species are more important to community resilience and biochemical functions.
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http://dx.doi.org/10.1038/s41586-019-1590-8DOI Listing
October 2019

Strategies in times of crisis-insights into the benthic foraminiferal record of the Palaeocene-Eocene Thermal Maximum.

Philos Trans A Math Phys Eng Sci 2018 Oct;376(2130)

School of Geographical Science, University of Bristol, University Road, Bristol BS8 1SS, UK.

Climate change is predicted to alter temperature, carbonate chemistry and oxygen availability in the oceans, which will affect individuals, populations and ecosystems. We use the fossil record of benthic foraminifers to assess developmental impacts in response to environmental changes during the Palaeocene-Eocene Thermal Maximum (PETM). Using an unprecedented number of µ-computed tomography scans, we determine the size of the proloculus (first chamber), the number of chambers and the final size of two benthic foraminiferal species which survived the extinction at sites 690 (Atlantic sector, Southern Ocean, palaeodepth 1900 m), 1210 (central equatorial Pacific, palaeodepth 2100 m) and 1135 (Indian Ocean sector, Southern Ocean, palaeodepth 600-1000 m). The population at the shallowest site, 1135, does not show a clear response to the PETM, whereas those at the other sites record reductions in diameter or proloculus size. Temperature was similar at all sites, thus it is not likely to be the reason for differences between sites. At site 1210, small size coincided with higher chamber numbers during the peak event, and may have been caused by a combination of low carbonate ion concentrations and low food supply. Dwarfing at site 690 occurred at lower chamber numbers, and may have been caused by decreasing carbonate saturation at sufficient food levels to reproduce. Proloculus size varied strongly between sites and through time, suggesting a large influence of environment on both microspheric and megalospheric forms without clear bimodality. The effect of the environmental changes during the PETM was more pronounced at deeper sites, possibly implicating carbonate saturation.This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.
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http://dx.doi.org/10.1098/rsta.2017.0328DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6127389PMC
October 2018

Late inception of a resiliently oxygenated upper ocean.

Science 2018 07 31;361(6398):174-177. Epub 2018 May 31.

Department of Earth Sciences, Syracuse University, Syracuse, NY, USA.

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.
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http://dx.doi.org/10.1126/science.aar5372DOI Listing
July 2018

Very large release of mostly volcanic carbon during the Palaeocene-Eocene Thermal Maximum.

Nature 2017 08;548(7669):573-577

Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton SO17 1BJ, UK.

The Palaeocene-Eocene Thermal Maximum (PETM) was a global warming event that occurred about 56 million years ago, and is commonly thought to have been driven primarily by the destabilization of carbon from surface sedimentary reservoirs such as methane hydrates. However, it remains controversial whether such reservoirs were indeed the source of the carbon that drove the warming. Resolving this issue is key to understanding the proximal cause of the warming, and to quantifying the roles of triggers versus feedbacks. Here we present boron isotope data-a proxy for seawater pH-that show that the ocean surface pH was persistently low during the PETM. We combine our pH data with a paired carbon isotope record in an Earth system model in order to reconstruct the unfolding carbon-cycle dynamics during the event. We find strong evidence for a much larger (more than 10,000 petagrams)-and, on average, isotopically heavier-carbon source than considered previously. This leads us to identify volcanism associated with the North Atlantic Igneous Province, rather than carbon from a surface reservoir, as the main driver of the PETM. This finding implies that climate-driven amplification of organic carbon feedbacks probably played only a minor part in driving the event. However, we find that enhanced burial of organic matter seems to have been important in eventually sequestering the released carbon and accelerating the recovery of the Earth system.
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http://dx.doi.org/10.1038/nature23646DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5582631PMC
August 2017

A probabilistic assessment of the rapidity of PETM onset.

Nat Commun 2017 08 25;8(1):353. Epub 2017 Aug 25.

Department of Earth Sciences, University of California, Riverside, Riverside, CA, 92506, USA.

Knowledge of the onset duration of the Paleocene-Eocene Thermal Maximum-the largest known greenhouse-gas-driven global warming event of the Cenozoic-is central to drawing inferences for future climate change. Single-foraminifera measurements of the associated carbon isotope excursion from Maud Rise (South Atlantic Ocean) are controversial, as they seem to indicate geologically instantaneous carbon release and anomalously long ocean mixing. Here, we fundamentally reinterpret this record and extract the likely PETM onset duration. First, we employ an Earth system model to illustrate how the response of ocean circulation to warming does not support the interpretation of instantaneous carbon release. Instead, we use a novel sediment-mixing model to show how changes in the relative population sizes of calcareous plankton, combined with sediment mixing, can explain the observations. Furthermore, for any plausible PETM onset duration and sampling methodology, we place a probability on not sampling an intermediate, syn-excursion isotopic value. Assuming mixed-layer carbonate production continued at Maud Rise, we deduce the PETM onset was likely <5 kyr.Single-foraminifera measurements of the PETM carbon isotope excursion from Maud Rise have been interpreted as indicating geologically instantaneous carbon release. Here, the authors explain these records using an Earth system model and a sediment-mixing model and extract the likely PETM onset duration.
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http://dx.doi.org/10.1038/s41467-017-00292-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5572461PMC
August 2017

Why marine phytoplankton calcify.

Sci Adv 2016 07 13;2(7):e1501822. Epub 2016 Jul 13.

School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, UK.; Department of Earth Sciences, University of California, Riverside, Riverside, CA 92521, USA.

Calcifying marine phytoplankton-coccolithophores- are some of the most successful yet enigmatic organisms in the ocean and are at risk from global change. To better understand how they will be affected, we need to know "why" coccolithophores calcify. We review coccolithophorid evolutionary history and cell biology as well as insights from recent experiments to provide a critical assessment of the costs and benefits of calcification. We conclude that calcification has high energy demands and that coccolithophores might have calcified initially to reduce grazing pressure but that additional benefits such as protection from photodamage and viral/bacterial attack further explain their high diversity and broad spectrum ecology. The cost-benefit aspect of these traits is illustrated by novel ecosystem modeling, although conclusive observations remain limited. In the future ocean, the trade-off between changing ecological and physiological costs of calcification and their benefits will ultimately decide how this important group is affected by ocean acidification and global warming.
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http://dx.doi.org/10.1126/sciadv.1501822DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4956192PMC
July 2016

Changing atmospheric CO2 concentration was the primary driver of early Cenozoic climate.

Nature 2016 05 25;533(7603):380-4. Epub 2016 Apr 25.

School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff CF10 3AT, UK.

The Early Eocene Climate Optimum (EECO, which occurred about 51 to 53 million years ago), was the warmest interval of the past 65 million years, with mean annual surface air temperature over ten degrees Celsius warmer than during the pre-industrial period. Subsequent global cooling in the middle and late Eocene epoch, especially at high latitudes, eventually led to continental ice sheet development in Antarctica in the early Oligocene epoch (about 33.6 million years ago). However, existing estimates place atmospheric carbon dioxide (CO2) levels during the Eocene at 500-3,000 parts per million, and in the absence of tighter constraints carbon-climate interactions over this interval remain uncertain. Here we use recent analytical and methodological developments to generate a new high-fidelity record of CO2 concentrations using the boron isotope (δ(11)B) composition of well preserved planktonic foraminifera from the Tanzania Drilling Project, revising previous estimates. Although species-level uncertainties make absolute values difficult to constrain, CO2 concentrations during the EECO were around 1,400 parts per million. The relative decline in CO2 concentration through the Eocene is more robustly constrained at about fifty per cent, with a further decline into the Oligocene. Provided the latitudinal dependency of sea surface temperature change for a given climate forcing in the Eocene was similar to that of the late Quaternary period, this CO2 decline was sufficient to drive the well documented high- and low-latitude cooling that occurred through the Eocene. Once the change in global temperature between the pre-industrial period and the Eocene caused by the action of all known slow feedbacks (apart from those associated with the carbon cycle) is removed, both the EECO and the late Eocene exhibit an equilibrium climate sensitivity relative to the pre-industrial period of 2.1 to 4.6 degrees Celsius per CO2 doubling (66 per cent confidence), which is similar to the canonical range (1.5 to 4.5 degrees Celsius), indicating that a large fraction of the warmth of the early Eocene greenhouse was driven by increased CO2 concentrations, and that climate sensitivity was relatively constant throughout this period.
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http://dx.doi.org/10.1038/nature17423DOI Listing
May 2016

Combustion of available fossil fuel resources sufficient to eliminate the Antarctic Ice Sheet.

Sci Adv 2015 Sep 11;1(8):e1500589. Epub 2015 Sep 11.

Department of Global Ecology, Carnegie Institution for Science, Stanford, CA 94305, USA.

The Antarctic Ice Sheet stores water equivalent to 58 m in global sea-level rise. We show in simulations using the Parallel Ice Sheet Model that burning the currently attainable fossil fuel resources is sufficient to eliminate the ice sheet. With cumulative fossil fuel emissions of 10,000 gigatonnes of carbon (GtC), Antarctica is projected to become almost ice-free with an average contribution to sea-level rise exceeding 3 m per century during the first millennium. Consistent with recent observations and simulations, the West Antarctic Ice Sheet becomes unstable with 600 to 800 GtC of additional carbon emissions. Beyond this additional carbon release, the destabilization of ice basins in both West and East Antarctica results in a threshold increase in global sea level. Unabated carbon emissions thus threaten the Antarctic Ice Sheet in its entirety with associated sea-level rise that far exceeds that of all other possible sources.
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http://dx.doi.org/10.1126/sciadv.1500589DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4643791PMC
September 2015

A neoproterozoic transition in the marine nitrogen cycle.

Curr Biol 2014 Mar 27;24(6):652-7. Epub 2014 Feb 27.

Division of Plant Science, University of Dundee at the James Hutton Institute, Invergowrie, Dundee DD2 5DA, UK; School of Plant Biology, University of Western Australia, Crawley, WA 6009, Australia.

The Neoproterozoic (1000-542 million years ago, Mya) was characterized by profound global environmental and evolutionary changes, not least of which included a major rise in atmospheric oxygen concentrations [1, 2], extreme climatic fluctuations and global-scale glaciation [3], and the emergence of metazoan life in the oceans [4, 5]. We present here phylogenomic (135 proteins and two ribosomal RNAs, SSU and LSU) and relaxed molecular clock (SSU, LSU, and rpoC1) analyses that identify this interval as a key transition in the marine nitrogen cycle. Specifically, we identify the Cryogenian (850-635 Mya) as heralding the first appearance of both marine planktonic unicellular nitrogen-fixing cyanobacteria and non-nitrogen-fixing picocyanobacteria (Synechococcus and Prochlorococcus [6]). Our findings are consistent with the existence of open-ocean environmental conditions earlier in the Proterozoic adverse to nitrogen-fixers and their evolution-specifically, insufficient availability of molybdenum and vanadium, elements essential to the production of high-yielding nitrogenases. As these elements became more abundant during the Cryogenian [7, 8], both nitrogen-fixing cyanobacteria and planktonic picocyanobacteria diversified. The subsequent emergence of a strong biological pump in the ocean implied by our evolutionary reconstruction may help in explaining increased oxygenation of the Earth's surface at this time, as well as tendency for glaciation.
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http://dx.doi.org/10.1016/j.cub.2014.01.041DOI Listing
March 2014

Onset of carbon isotope excursion at the Paleocene-Eocene thermal maximum took millennia, not 13 years.

Proc Natl Acad Sci U S A 2014 Mar 26;111(12):E1062-3. Epub 2014 Feb 26.

Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822.

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http://dx.doi.org/10.1073/pnas.1321177111DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3970517PMC
March 2014

Future habitat suitability for coral reef ecosystems under global warming and ocean acidification.

Glob Chang Biol 2013 Dec 8;19(12):3592-606. Epub 2013 Oct 8.

School of Geographical Sciences, University of Bristol, Bristol, BS8 1SS, UK; School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK.

Rising atmospheric CO2 concentrations are placing spatially divergent stresses on the world's tropical coral reefs through increasing ocean surface temperatures and ocean acidification. We show how these two stressors combine to alter the global habitat suitability for shallow coral reef ecosystems, using statistical Bioclimatic Envelope Models rather than basing projections on any a priori assumptions of physiological tolerances or fixed thresholds. We apply two different modeling approaches (Maximum Entropy and Boosted Regression Trees) with two levels of complexity (one a simplified and reduced environmental variable version of the other). Our models project a marked temperature-driven decline in habitat suitability for many of the most significant and bio-diverse tropical coral regions, particularly in the central Indo-Pacific. This is accompanied by a temperature-driven poleward range expansion of favorable conditions accelerating up to 40-70 km per decade by 2070. We find that ocean acidification is less influential for determining future habitat suitability than warming, and its deleterious effects are centered evenly in both hemispheres between 5° and 20° latitude. Contrary to expectations, the combined impact of ocean surface temperature rise and acidification leads to little, if any, degradation in future habitat suitability across much of the Atlantic and areas currently considered 'marginal' for tropical corals, such as the eastern Equatorial Pacific. These results are consistent with fossil evidence of range expansions during past warm periods. In addition, the simplified models are particularly sensitive to short-term temperature variations and their projections correlate well with reported locations of bleaching events. Our approach offers new insights into the relative impact of two global environmental pressures associated with rising atmospheric CO2 on potential future habitats, but greater understanding of past and current controls on coral reef ecosystems is essential to their conservation and management under a changing climate.
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http://dx.doi.org/10.1111/gcb.12335DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4028991PMC
December 2013

Surviving rapid climate change in the deep sea during the Paleogene hyperthermals.

Proc Natl Acad Sci U S A 2013 Jun 20;110(23):9273-6. Epub 2013 May 20.

Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, United Kingdom.

Predicting the impact of ongoing anthropogenic CO2 emissions on calcifying marine organisms is complex, owing to the synergy between direct changes (acidification) and indirect changes through climate change (e.g., warming, changes in ocean circulation, and deoxygenation). Laboratory experiments, particularly on longer-lived organisms, tend to be too short to reveal the potential of organisms to acclimatize, adapt, or evolve and usually do not incorporate multiple stressors. We studied two examples of rapid carbon release in the geological record, Eocene Thermal Maximum 2 (∼53.2 Ma) and the Paleocene Eocene Thermal Maximum (PETM, ∼55.5 Ma), the best analogs over the last 65 Ma for future ocean acidification related to high atmospheric CO2 levels. We use benthic foraminifers, which suffered severe extinction during the PETM, as a model group. Using synchrotron radiation X-ray tomographic microscopy, we reconstruct the calcification response of survivor species and find, contrary to expectations, that calcification significantly increased during the PETM. In contrast, there was no significant response to the smaller Eocene Thermal Maximum 2, which was associated with a minor change in diversity only. These observations suggest that there is a response threshold for extinction and calcification response, while highlighting the utility of the geological record in helping constrain the sensitivity of biotic response to environmental change.
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http://dx.doi.org/10.1073/pnas.1300579110DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3677492PMC
June 2013

A Cenozoic record of the equatorial Pacific carbonate compensation depth.

Nature 2012 Aug;488(7413):609-14

Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Waterfront Campus, European Way, Southampton SO14 3ZH, UK.

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0-3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.
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http://dx.doi.org/10.1038/nature11360DOI Listing
August 2012

Geoengineering: taking control of our planet's climate?

Philos Trans A Math Phys Eng Sci 2012 Sep;370(1974):4163-5

School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, UK.

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http://dx.doi.org/10.1098/rsta.2012.0290DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3405668PMC
September 2012

The geological record of ocean acidification.

Science 2012 Mar;335(6072):1058-63

Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA.

Ocean acidification may have severe consequences for marine ecosystems; however, assessing its future impact is difficult because laboratory experiments and field observations are limited by their reduced ecologic complexity and sample period, respectively. In contrast, the geological record contains long-term evidence for a variety of global environmental perturbations, including ocean acidification plus their associated biotic responses. We review events exhibiting evidence for elevated atmospheric CO(2), global warming, and ocean acidification over the past ~300 million years of Earth's history, some with contemporaneous extinction or evolutionary turnover among marine calcifiers. Although similarities exist, no past event perfectly parallels future projections in terms of disrupting the balance of ocean carbonate chemistry-a consequence of the unprecedented rapidity of CO(2) release currently taking place.
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http://dx.doi.org/10.1126/science.1208277DOI Listing
March 2012

Evolution of the ocean's "biological pump".

Authors:
Andy Ridgwell

Proc Natl Acad Sci U S A 2011 Oct 26;108(40):16485-6. Epub 2011 Sep 26.

Bristol Research Initiative for the Dynamic Global Environment, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, United Kingdom.

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http://dx.doi.org/10.1073/pnas.1112236108DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3189068PMC
October 2011

Are there pre-Quaternary geological analogues for a future greenhouse warming?

Philos Trans A Math Phys Eng Sci 2011 Mar;369(1938):933-56

School of Earth and Environment, Earth and Environment Building, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.

Given the inherent uncertainties in predicting how climate and environments will respond to anthropogenic emissions of greenhouse gases, it would be beneficial to society if science could identify geological analogues to the human race's current grand climate experiment. This has been a focus of the geological and palaeoclimate communities over the last 30 years, with many scientific papers claiming that intervals in Earth history can be used as an analogue for future climate change. Using a coupled ocean-atmosphere modelling approach, we test this assertion for the most probable pre-Quaternary candidates of the last 100 million years: the Mid- and Late Cretaceous, the Palaeocene-Eocene Thermal Maximum (PETM), the Early Eocene, as well as warm intervals within the Miocene and Pliocene epochs. These intervals fail as true direct analogues since they either represent equilibrium climate states to a long-term CO(2) forcing--whereas anthropogenic emissions of greenhouse gases provide a progressive (transient) forcing on climate--or the sensitivity of the climate system itself to CO(2) was different. While no close geological analogue exists, past warm intervals in Earth history provide a unique opportunity to investigate processes that operated during warm (high CO(2)) climate states. Palaeoclimate and environmental reconstruction/modelling are facilitating the assessment and calculation of the response of global temperatures to increasing CO(2) concentrations in the longer term (multiple centuries); this is now referred to as the Earth System Sensitivity, which is critical in identifying CO(2) thresholds in the atmosphere that must not be crossed to avoid dangerous levels of climate change in the long term. Palaeoclimatology also provides a unique and independent way to evaluate the qualities of climate and Earth system models used to predict future climate.
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http://dx.doi.org/10.1098/rsta.2010.0317DOI Listing
March 2011

'Geoengineering'--taking control of our planet's climate.

Sci Prog 2009 ;92(Pt 2):139-62

University of Bristol, School of Geographical Sciences, University Road, Bristol BS8 ISS, UK.

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.
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http://dx.doi.org/10.3184/003685009X461495DOI Listing
September 2009

Climate and climate change.

Curr Biol 2009 Jul;19(14):R563-6

Bristol Research Initiative for the Dynamic Global Environment, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK.

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http://dx.doi.org/10.1016/j.cub.2009.05.014DOI Listing
July 2009

Tackling regional climate change by leaf albedo bio-geoengineering.

Curr Biol 2009 Jan 15;19(2):146-50. Epub 2009 Jan 15.

Bristol Research Initiative for the Dynamic Global Environment, School of Geographical Sciences, University of Bristol, Bristol BS81SS, UK.

The likelihood that continuing greenhouse-gas emissions will lead to an unmanageable degree of climate change has stimulated the search for planetary-scale technological solutions for reducing global warming ("geoengineering"), typically characterized by the necessity for costly new infrastructures and industries. We suggest that the existing global infrastructure associated with arable agriculture can help, given that crop plants exert an important influence over the climatic energy budget because of differences in their albedo (solar reflectivity) compared to soils and to natural vegetation. Specifically, we propose a "bio-geoengineering" approach to mitigate surface warming, in which crop varieties having specific leaf glossiness and/or canopy morphological traits are specifically chosen to maximize solar reflectivity. We quantify this by modifying the canopy albedo of vegetation in prescribed cropland areas in a global-climate model, and thereby estimate the near-term potential for bio-geoengineering to be a summertime cooling of more than 1 degrees C throughout much of central North America and midlatitude Eurasia, equivalent to seasonally offsetting approximately one-fifth of regional warming due to doubling of atmospheric CO(2). Ultimately, genetic modification of plant leaf waxes or canopy structure could achieve greater temperature reductions, although better characterization of existing intraspecies variability is needed first.
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http://dx.doi.org/10.1016/j.cub.2008.12.025DOI Listing
January 2009

Carbonate deposition, climate stability, and Neoproterozoic ice ages.

Science 2003 Oct;302(5646):859-62

Department of Earth Sciences, University of California-Riverside, Riverside, CA 92521, USA.

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 CO2-climate feedback on ice-sheet growth.
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http://dx.doi.org/10.1126/science.1088342DOI Listing
October 2003

Dust in the Earth system: the biogeochemical linking of land, air and sea.

Authors:
Andy J Ridgwell

Philos Trans A Math Phys Eng Sci 2002 Dec;360(1801):2905-24

School of Environmental Sciences and Tyndall Centre for Climate Change Research, University of East Anglia, Norwich NR4 7TJ, UK.

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 CO(2) 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.
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http://dx.doi.org/10.1098/rsta.2002.1096DOI Listing
December 2002