Final Report Summary - PALEOCARB (Role of the marine carbon cycle in the climate system)
The general topic of this proposal was the study of the marine carbon cycle and its role in the climate system. The overall objective was to investigate the main processes driving the export of organic matter (OM) to the deep sea and marine sediments, and assess their role inbiogeochemical processes of the marine carbon cycle, within a physical oceanographic and climatic context. The goal of the project was to obtain quantitative information on marine carbon cycle processes using an interdisciplinary approach. The project had a global scope and it focussed on timescales from the Pliocene (prior to 2. 6 milion years ago to present. The approach relied in determining the chemical composition of sediments (biomarkers) to identify the various sources of OM inputs and their fluxes to sediments and compare the data to modelling results.
The main results and conclusions of the project:
In relation to changes in the biogeochemical cycles since the Pliocene, in a paper in the journal Nature it was described how dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean. Indeed, dust supply to the Southern Ocean increases during ice ages, and'iron fertilisation'of the subantarctic zone may have contributed up to 40 parts per million by volume (p. p. m. v.) of the decrease (80100 p. p. m. v.) in atmospheric carbon dioxide observed during late Pleistocene glacial cycles. So far, however, the magnitude of Southern Ocean dust deposition in earlier times and its role in the development and evolution of Pleistocene glacial cycles have remained unclear. We reported a high-resolution record of dust and iron supply to the Southern Ocean over the past four million years, derived from the analysis of marine sediments from ODP Site 1090, located in the Atlantic sector of the subantarctic zone. The close correspondence of our dust and iron deposition records with Antarctic ice core reconstructions of dust flux covering the past 800, 000 yearsindicates that both of these archives record large-scale deposition changes that should apply to most of the Southern Ocean, validating previous interpretations of the ice core data. The extension of the record beyond the interval covered by the Antarctic ice cores reveals that, in contrast to the relatively gradual intensification of glacial cycles over the past three million years, Southern Ocean dust and iron flux rose sharply at the Mid-Pleistocene climatic transition around 1. 25 million years ago. This finding complements previous observations over late Pleistocene glacial cycles, providing new evidence of a tight connection between high dust input to the Southern Ocean and the emergence of the deep glaciations that characterise the past one million years of Earth history.
In relation to using paleodata to constrain the effect of CO2 on climate I publish in the journal Science how to improve reliable assessment of future impacts of anthropogenic carbon emissions on Earth's systems and human welfare by reducing uncertainties in our understanding of the equilibrium climate system sensitivity. This parameter can be generally defined as a change in global mean surface air temperature ?T caused by an arbitrary perturbation ?F (radiative forcing) of Earth's radiative balance at the top of the atmosphere with respect to a given reference state. The equilibrium climate sensitivity for a doubling of atmospheric CO2 concentrations from a preindustrial (1840-1850 AD) reference state (ECS2xC) has been established as a well-defined standard measure. Because transient climate change typically scales with ECS2xC it is a useful and important diagnostic in climate modeling. Initial estimates of ECS2xC = 3±1. 5 K suggested a large uncertainty, which has not been reduced in the last 32 years despite considerable efforts. To the contrary, many recent studies suggested the possibility of very high (up to 10 K and higher) values for ECS2xC. Efforts to use observations from the last 150 years to constrain the upper end of ECS2xC have proven futile, because of the low signal to noise ratio and uncertainties associated with aerosol forcing. Data from the LGM are particularly useful to estimate ECS2xC because large differences with pre-industrial climate lead to a favorable signal to noise ratio, and both radiative forcings and surface temperatures are well constrained from extensive paleoclimate reconstructions. My colleagues and I used a a spatially extensive network of paleoclimate observations from the Last Glacial Maximum (LGM, 21, 000 years ago) in combination with model simulations to estimate a probability distribution of ECS2xC = 2. 5 K with a likely range of 1. 6-3. 6 K, and < 5 % chance of sensitivities larger than 4. 4 K. We found that climate sensitivities larger than 6 K are implausible, and that both the most likely value and the uncertainty range are smaller than previously thought. This demonstrated that paleoclimate data provide efficient constraints to reduce the uncertainty of future climate projections.
I also investigated how to improve on the use of certain biomarker proxies reconstruct past marine carbon cycle parameters. In the paper published in the journal Limnology and Oceanography we showed that the concentrations of chlorophyll transformation products indicative of phytoplankton production and crenarchaeol (a marker for Crenarchaea abundance, i. e. Archaea) are significantly positively correlated (Spearman's rank correlation coefficient rs > 0. 75) in four core records from freshwater (Lake Baikal) and marine settings (Southern, Atlantic, and Arctic Oceans). This suggests a close relationship between Crenarchaea abundance and phytoplankton production. Degradation and transport mechanisms as well as a common environmental trigger may in part account for our observations, but these mechanisms alone cannot fully explain them. Instead our findings point to a metabolic dependence of Crenarchaea on resources released by phytoplankton, such as organic carbon or ammonium.
In another paper still in evaluation in Quaternary Science Reviews we address the issue of the seasonality of carbon fluxes by studying their influence in the application of alkenone-UK37'index. Using a compilation of 28 sediment trap studies that measured alkenone fluxes we show that simple paradigms on alkenone sedimentation patterns are not forthcoming. Thus, we find that the seasonality of maximum alkenone fluxes in sediment traps vary markedly across the oceans, depending on the characteristics of the local oceanographic settings, rather than driven just by latitude and light availability as could be inferred from net primary productivity patterns. The seasonality of the fluxes to sediments may also be altered due to the complexity of sedimentation processes. The end result is that although the UK37'export signals in some locations are seasonally biased, the sedimentary values of UK37'may as well represent annual SST averages, as also inferred from the field calibrations of the proxy. A single consistent pattern does not apply universally.
Potential impact
The response of the climate system to increasing concentrations of greenhouse gases is of primary concern to human societies. Yet, forecasts of the sign, amplitude and spatial pattern of this response remain associated with significant uncertainty. A better understanding of the climate system is limited by the lack of direct observational data on its behaviour under different boundary conditions.comparison of paleo-proxy data with past climate simulations by coupled atmosphere-ocean GCMs can reveal to what degree the output of the most comprehensive tools for predicting future climate changes is consistent with the evidence from the past. However, a robust model-data comparison requires reliable global-scale past climate reconstructions. But, systematic instrumental measurements of ocean properties exist for only a few decades, with the longest regional records rarely extending beyond the 19th century. Yet, it is only with the aid of climate records spanning thousands of years and encompassing dramatically different climatic states of the planet that one can truly understand the dynamics of the oceanatmosphere interface and perform meaningful and useful tests of global climate models. Information on the state of the planet in the past, and the amplitude, frequency and mechanisms of its changes is of paramount importance to our society, as it is used to inform and guide long term environmental policies and planning and to predict impact of climate change on land, our habitat.
The main results and conclusions of the project:
In relation to changes in the biogeochemical cycles since the Pliocene, in a paper in the journal Nature it was described how dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean. Indeed, dust supply to the Southern Ocean increases during ice ages, and'iron fertilisation'of the subantarctic zone may have contributed up to 40 parts per million by volume (p. p. m. v.) of the decrease (80100 p. p. m. v.) in atmospheric carbon dioxide observed during late Pleistocene glacial cycles. So far, however, the magnitude of Southern Ocean dust deposition in earlier times and its role in the development and evolution of Pleistocene glacial cycles have remained unclear. We reported a high-resolution record of dust and iron supply to the Southern Ocean over the past four million years, derived from the analysis of marine sediments from ODP Site 1090, located in the Atlantic sector of the subantarctic zone. The close correspondence of our dust and iron deposition records with Antarctic ice core reconstructions of dust flux covering the past 800, 000 yearsindicates that both of these archives record large-scale deposition changes that should apply to most of the Southern Ocean, validating previous interpretations of the ice core data. The extension of the record beyond the interval covered by the Antarctic ice cores reveals that, in contrast to the relatively gradual intensification of glacial cycles over the past three million years, Southern Ocean dust and iron flux rose sharply at the Mid-Pleistocene climatic transition around 1. 25 million years ago. This finding complements previous observations over late Pleistocene glacial cycles, providing new evidence of a tight connection between high dust input to the Southern Ocean and the emergence of the deep glaciations that characterise the past one million years of Earth history.
In relation to using paleodata to constrain the effect of CO2 on climate I publish in the journal Science how to improve reliable assessment of future impacts of anthropogenic carbon emissions on Earth's systems and human welfare by reducing uncertainties in our understanding of the equilibrium climate system sensitivity. This parameter can be generally defined as a change in global mean surface air temperature ?T caused by an arbitrary perturbation ?F (radiative forcing) of Earth's radiative balance at the top of the atmosphere with respect to a given reference state. The equilibrium climate sensitivity for a doubling of atmospheric CO2 concentrations from a preindustrial (1840-1850 AD) reference state (ECS2xC) has been established as a well-defined standard measure. Because transient climate change typically scales with ECS2xC it is a useful and important diagnostic in climate modeling. Initial estimates of ECS2xC = 3±1. 5 K suggested a large uncertainty, which has not been reduced in the last 32 years despite considerable efforts. To the contrary, many recent studies suggested the possibility of very high (up to 10 K and higher) values for ECS2xC. Efforts to use observations from the last 150 years to constrain the upper end of ECS2xC have proven futile, because of the low signal to noise ratio and uncertainties associated with aerosol forcing. Data from the LGM are particularly useful to estimate ECS2xC because large differences with pre-industrial climate lead to a favorable signal to noise ratio, and both radiative forcings and surface temperatures are well constrained from extensive paleoclimate reconstructions. My colleagues and I used a a spatially extensive network of paleoclimate observations from the Last Glacial Maximum (LGM, 21, 000 years ago) in combination with model simulations to estimate a probability distribution of ECS2xC = 2. 5 K with a likely range of 1. 6-3. 6 K, and < 5 % chance of sensitivities larger than 4. 4 K. We found that climate sensitivities larger than 6 K are implausible, and that both the most likely value and the uncertainty range are smaller than previously thought. This demonstrated that paleoclimate data provide efficient constraints to reduce the uncertainty of future climate projections.
I also investigated how to improve on the use of certain biomarker proxies reconstruct past marine carbon cycle parameters. In the paper published in the journal Limnology and Oceanography we showed that the concentrations of chlorophyll transformation products indicative of phytoplankton production and crenarchaeol (a marker for Crenarchaea abundance, i. e. Archaea) are significantly positively correlated (Spearman's rank correlation coefficient rs > 0. 75) in four core records from freshwater (Lake Baikal) and marine settings (Southern, Atlantic, and Arctic Oceans). This suggests a close relationship between Crenarchaea abundance and phytoplankton production. Degradation and transport mechanisms as well as a common environmental trigger may in part account for our observations, but these mechanisms alone cannot fully explain them. Instead our findings point to a metabolic dependence of Crenarchaea on resources released by phytoplankton, such as organic carbon or ammonium.
In another paper still in evaluation in Quaternary Science Reviews we address the issue of the seasonality of carbon fluxes by studying their influence in the application of alkenone-UK37'index. Using a compilation of 28 sediment trap studies that measured alkenone fluxes we show that simple paradigms on alkenone sedimentation patterns are not forthcoming. Thus, we find that the seasonality of maximum alkenone fluxes in sediment traps vary markedly across the oceans, depending on the characteristics of the local oceanographic settings, rather than driven just by latitude and light availability as could be inferred from net primary productivity patterns. The seasonality of the fluxes to sediments may also be altered due to the complexity of sedimentation processes. The end result is that although the UK37'export signals in some locations are seasonally biased, the sedimentary values of UK37'may as well represent annual SST averages, as also inferred from the field calibrations of the proxy. A single consistent pattern does not apply universally.
Potential impact
The response of the climate system to increasing concentrations of greenhouse gases is of primary concern to human societies. Yet, forecasts of the sign, amplitude and spatial pattern of this response remain associated with significant uncertainty. A better understanding of the climate system is limited by the lack of direct observational data on its behaviour under different boundary conditions.comparison of paleo-proxy data with past climate simulations by coupled atmosphere-ocean GCMs can reveal to what degree the output of the most comprehensive tools for predicting future climate changes is consistent with the evidence from the past. However, a robust model-data comparison requires reliable global-scale past climate reconstructions. But, systematic instrumental measurements of ocean properties exist for only a few decades, with the longest regional records rarely extending beyond the 19th century. Yet, it is only with the aid of climate records spanning thousands of years and encompassing dramatically different climatic states of the planet that one can truly understand the dynamics of the oceanatmosphere interface and perform meaningful and useful tests of global climate models. Information on the state of the planet in the past, and the amplitude, frequency and mechanisms of its changes is of paramount importance to our society, as it is used to inform and guide long term environmental policies and planning and to predict impact of climate change on land, our habitat.