scholarly journals Oligocene–Miocene paleoceanography off the Wilkes Land Margin (East Antarctica) based on organic-walled dinoflagellate cysts

2017 ◽  
Author(s):  
Peter K. Bijl ◽  
Alexander J. P. Houben ◽  
Julian D. Hartman ◽  
Jörg Pross ◽  
Ariadna Salabarnada ◽  
...  
Keyword(s):  
Sea Ice ◽  
Ice Sheet ◽  
Critical Factor ◽  
East Antarctica ◽  
Atmospheric Co2 ◽  
Time Intervals ◽  
Integrated Ocean Drilling Program ◽  
Co2 Concentrations ◽  
Wilkes Land ◽  
Atmospheric Co2 Concentrations

Abstract. Next to atmospheric CO2 concentrations, oceanographic conditions are a critical factor determining the stability of Antarctic marine-terminating ice sheets. The Oligocene and Miocene epochs (~ 34–5 Ma) were time intervals with atmospheric CO2 concentrations between those of present-day and those expected for the near future. As such, these time intervals may bear information to resolve the uncertainties that still exist in the projection of future ice-sheet volume decline. We present organic-walled dinoflagellate cyst (dinocyst) assemblages from chronostratigraphically well-constrained Oligocene to mid-Miocene sediments from Integrated Ocean Drilling Program Expedition (IODP) Site U1356. Situated offshore the Wilkes Land continental margin, East Antarctica, the sediment core has archived past dynamics of an ice sheet that is today mostly grounded below sea level. We interpret dinocyst assemblages in terms of paleoceanographic change on different time scales, i.e., on glacial-interglacial and long-term variability. Sea-ice indicators occur only for the first 1.5 Ma following the full Antarctic continental glaciation during the early Oligocene, and after the Middle Miocene Climatic Optimum. During the remainder of the Oligocene and Miocene dinocysts suggest a weaker-than-modern sea-ice season. The assemblages generally bear strong similarity to present-day open-ocean, high-nutrient settings north of the sea ice edge, with episodic dominance of temperate species similar to the present-day subtropical front. Oligotrophic and temperate surface waters prevailed over the site notably during interglacial time intervals, suggesting that the position of the (subpolar) oceanic frontal systems have varied in concordance with Oligocene-Miocene glacial-interglacial climate variability.

scholarly journals Paleoceanography and ice sheet variability offshore Wilkes Land, Antarctica – Part 2: Insights from Oligocene–Miocene dinoflagellate cyst assemblages

2018 ◽  
Vol 14 (7) ◽  
pp. 1015-1033 ◽  
Author(s):  
Peter K. Bijl ◽  
Alexander J. P. Houben ◽  
Julian D. Hartman ◽  
Jörg Pross ◽  
Ariadna Salabarnada ◽  
...  
Keyword(s):  
Sea Ice ◽  
Ice Sheet ◽  
Critical Factor ◽  
Atmospheric Co2 ◽  
Dinoflagellate Cyst ◽  
Volume Stability ◽  
Integrated Ocean Drilling Program ◽  
Co2 Concentrations ◽  
Wilkes Land ◽  
Atmospheric Co2 Concentrations

Abstract. Next to atmospheric CO2 concentrations, ice-proximal oceanographic conditions are a critical factor for the stability of Antarctic marine-terminating ice sheets. The Oligocene and Miocene epochs (∼ 34–5 Myr ago) were time intervals with atmospheric CO2 concentrations between those of present-day and those expected for the near future. As such, these past analogues may provide insights into ice-sheet volume stability under warmer-than-present-day climates. We present organic-walled dinoflagellate cyst (dinocyst) assemblages from chronostratigraphically well-constrained Oligocene to mid-Miocene sediments from Integrated Ocean Drilling Program (IODP) Site U1356. Situated offshore the Wilkes Land continental margin, East Antarctica, the sediments from Site U1356 have archived the dynamics of an ice sheet that is today mostly grounded below sea level. We interpret dinocyst assemblages in terms of paleoceanographic change on different timescales, i.e. with regard to both glacial–interglacial and long-term variability. Our record shows that a sea-ice-related dinocyst species, Selenopemphix antarctica, occurs only for the first 1.5 Myr of the early Oligocene, following the onset of full continental glaciation on Antarctica, and after the Mid-Miocene Climatic Optimum. Dinocysts suggest a weaker-than-modern sea-ice season for the remainder of the Oligocene and Miocene. The assemblages generally bear strong similarity to present-day open-ocean, high-nutrient settings north of the sea-ice edge, with episodic dominance of temperate species similar to those found in the present-day subtropical front. Oligotrophic and temperate surface waters prevailed over the site notably during interglacial times, suggesting that the positions of the (subpolar) oceanic frontal systems have varied in concordance with Oligocene–Miocene glacial–interglacial climate variability.

scholarly journals Heterogeneous CO<sub>2</sub> and CH<sub>4</sub> content of glacial meltwater from the Greenland Ice Sheet and implications for subglacial carbon processes

2021 ◽  
Vol 15 (3) ◽  
pp. 1627-1644
Author(s):  
Andrea J. Pain ◽  
Jonathan B. Martin ◽  
Ellen E. Martin ◽  
Åsa K. Rennermalm ◽  
Shaily Rahman
Keyword(s):  
Greenhouse Gas ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Mineral Weathering ◽  
Atmospheric Co2 ◽  
The Arctic ◽  
Co2 Concentrations ◽  
Geochemical Models ◽  
Atmospheric Co2 Concentrations ◽  
The Impact

Abstract. Accelerated melting of the Greenland Ice Sheet has increased freshwater delivery to the Arctic Ocean and amplified the need to understand the impact of Greenland Ice Sheet meltwater on Arctic greenhouse gas budgets. We evaluate subglacial discharge from the Greenland Ice Sheet for carbon dioxide (CO2) and methane (CH4) concentrations and δ13C values and use geochemical models to evaluate subglacial CH4 and CO2 sources and sinks. We compare discharge from southwest (a sub-catchment of the Isunnguata Glacier, sub-Isunnguata, and the Russell Glacier) and southern Greenland (Kiattut Sermiat). Meltwater CH4 concentrations vary by orders of magnitude between sites and are saturated with respect to atmospheric concentrations at Kiattut Sermiat. In contrast, meltwaters from southwest sites are supersaturated, even though oxidation reduces CH4 concentrations by up to 50 % during periods of low discharge. CO2 concentrations range from supersaturated at sub-Isunnguata to undersaturated at Kiattut Sermiat. CO2 is consumed by mineral weathering throughout the melt season at all sites; however, differences in the magnitude of subglacial CO2 sources result in meltwaters that are either sources or sinks of atmospheric CO2. At the sub-Isunnguata site, the predominant source of CO2 is organic matter (OM) remineralization. However, multiple or heterogeneous subglacial CO2 sources maintain atmospheric CO2 concentrations at Russell but not at Kiattut Sermiat, where CO2 is undersaturated. These results highlight a previously unrecognized degree of heterogeneity in greenhouse gas dynamics under the Greenland Ice Sheet. Future work should constrain the extent and controls of heterogeneity to improve our understanding of the impact of Greenland Ice Sheet melt on Arctic greenhouse gas budgets, as well as the role of continental ice sheets in greenhouse gas variations over glacial–interglacial timescales.

scholarly journals Response of the carbon cycle in an intermediate complexity model to the different climate configurations of the last nine interglacials

2018 ◽  
Vol 14 (2) ◽  
pp. 239-253 ◽  
Author(s):  
Nathaelle Bouttes ◽  
Didier Swingedouw ◽  
Didier M. Roche ◽  
Maria F. Sanchez-Goni ◽  
Xavier Crosta
Keyword(s):  
Carbon Cycle ◽  
Ocean Circulation ◽  
Ice Sheet ◽  
Atmospheric Co2 ◽  
Oceanic Circulation ◽  
Intermediate Complexity ◽  
Co2 Concentrations ◽  
Co2 Levels ◽  
Atmospheric Co2 Concentrations ◽  
The Impact

Abstract. Atmospheric CO2 levels during interglacials prior to the Mid-Brunhes Event (MBE, ∼ 430 ka BP) were around 40 ppm lower than after the MBE. The reasons for this difference remain unclear. A recent hypothesis proposed that changes in oceanic circulation, in response to different external forcings before and after the MBE, might have increased the ocean carbon storage in pre-MBE interglacials, thus lowering atmospheric CO2. Nevertheless, no quantitative estimate of this hypothesis has been produced up to now. Here we use an intermediate complexity model including the carbon cycle to evaluate the response of the carbon reservoirs in the atmosphere, ocean and land in response to the changes of orbital forcings, ice sheet configurations and atmospheric CO2 concentrations over the last nine interglacials. We show that the ocean takes up more carbon during pre-MBE interglacials in agreement with data, but the impact on atmospheric CO2 is limited to a few parts per million. Terrestrial biosphere is simulated to be less developed in pre-MBE interglacials, which reduces the storage of carbon on land and increases atmospheric CO2. Accounting for different simulated ice sheet extents modifies the vegetation cover and temperature, and thus the carbon reservoir distribution. Overall, atmospheric CO2 levels are lower during these pre-MBE simulated interglacials including all these effects, but the magnitude is still far too small. These results suggest a possible misrepresentation of some key processes in the model, such as the magnitude of ocean circulation changes, or the lack of crucial mechanisms or internal feedbacks, such as those related to permafrost, to fully account for the lower atmospheric CO2 concentrations during pre-MBE interglacials.

scholarly journals Uncertainties in the modelled CO<sub>2</sub> threshold for Antarctic glaciation

2013 ◽  
Vol 9 (5) ◽  
pp. 5701-5745 ◽  
Author(s):  
E. Gasson ◽  
D. J. Lunt ◽  
R. DeConto ◽  
A. Goldner ◽  
M. Heinemann ◽  
...  
Keyword(s):  
Climate Models ◽  
Climate Model ◽  
Ice Sheet ◽  
Atmospheric Co2 ◽  
Lapse Rate ◽  
Proxy Records ◽  
Co2 Concentrations ◽  
Model Configuration ◽  
Model Dependency ◽  
Atmospheric Co2 Concentrations

Abstract. A frequently cited atmospheric CO2 threshold for the onset of Antarctic glaciation of ~ 780 ppmv is based on a study using an ice sheet model and the GENESIS climate model. Proxy records suggest that atmospheric CO2 concentrations passed through this threshold across the Eocene–Oligocene transition ~ 34 Ma. However, atmospheric CO2 concentrations may have been close to this threshold earlier than this transition, which is used by some to suggest the possibility of Antarctic ice sheets during the Eocene. Here we investigate the climate model dependency of the threshold for Antarctic glaciation by performing offline ice sheet model simulations using the climate from a number of different climate models (HadCM3L, CCSM3, CESM1.0, GENESIS, FAMOUS, ECHAM5 and GISS_ER). These climate simulations are sourced from a number of independent studies, as such the boundary conditions, which are poorly constrained during the Eocene, are not identical between simulations. The results of this study suggest that the atmospheric CO2 threshold for Antarctic glaciation is highly dependent on the climate model used and the climate model configuration. A large discrepancy between the climate model and ice sheet model grids for some simulations leads to a strong sensitivity to the lapse rate parameter. However, with the exception of HadCM3L and its reduced complexity version FAMOUS, the simulations suggest the growth of an intermediate sized ice sheet (> 25 m sea level equivalent) for atmospheric CO2 concentrations in the range of 560–920 ppmv, which is consistent with previous studies.

scholarly journals Response of the carbon cycle to the different orbital configurations of the last 9 interglacials

2016 ◽  
Author(s):  
Nathaelle Bouttes ◽  
Didier Swingedouw ◽  
Didier Roche ◽  
Maria Sanchez-Goni ◽  
Xavier Crosta
Keyword(s):  
Carbon Cycle ◽  
Ocean Circulation ◽  
Ice Sheet ◽  
Atmospheric Co2 ◽  
Oceanic Circulation ◽  
Co2 Concentrations ◽  
Before And After ◽  
Ocean Carbon ◽  
Atmospheric Co2 Concentrations ◽  
The Impact

Abstract. Atmospheric CO2 levels during interglacials prior to the Mid Bruhnes Event (MBE, ~ 430 ka BP) have lower values of around 40 ppm than after the MBE. The reasons for this difference remain unclear. A recent hypothesis proposed that changes in oceanic circulation, in response to differences in external forcing before and after the MBE, might have increased the ocean carbon storage and thus explained the lower CO2. Nevertheless, no quantitative estimate of this hypothesis has been produced up to now. Here we use an intermediate complexity model including the carbon cycle to evaluate the response of the carbon reservoirs in the atmosphere, ocean and land in response to the changes of orbital forcings and atmospheric CO2 concentrations over the nine last interglacials. We show that the ocean takes up more carbon during pre-MBE interglacials in agreement with data, but the impact on atmospheric CO2 is limited to a few ppm. Terrestrial biosphere is simulated to be less developed in pre-MBE interglacials, which reduces the storage of carbon on land and increases atmospheric CO2. Accounting for different simulated ice sheet extents modifies the vegetation cover and temperature, and thus the carbon reservoir distribution. Overall, atmospheric CO2 is slightly smaller in these pre-MBE simulated interglacials including ice sheet variations, but the magnitude is still far too small. These results suggest a possible mis-representation of some key processes in the model, such as the magnitude of ocean circulation changes, or the lack of crucial mechanisms or internal feedbacks, such as those related to permafrost, that could explain the lower atmospheric CO2 concentrations during pre-MBE interglacials.

scholarly journals Stratigraphic calibration of Oligocene–Miocene organic-walled dinoflagellate cysts from offshore Wilkes Land, East Antarctica, and a zonation proposal

2018 ◽  
Vol 37 (1) ◽  
pp. 105-138 ◽  
Author(s):  
Peter K. Bijl ◽  
Alexander J. P. Houben ◽  
Anja Bruls ◽  
Jörg Pross ◽  
Francesca Sangiorgi
Keyword(s):  
Southern Ocean ◽  
A Priori ◽  
East Antarctica ◽  
Dinoflagellate Cysts ◽  
Ocean Drilling Program ◽  
Integrated Ocean Drilling Program ◽  
Wilkes Land ◽  
Calcareous Nanoplankton ◽  
Atmospheric Co2 Concentrations

Abstract. There is growing interest in the scientific community in reconstructing the paleoceanography of the Southern Ocean during the Oligocene–Miocene because these time intervals experienced atmospheric CO2 concentrations with relevance to our future. However, it has remained notoriously difficult to put the sedimentary archives used in these efforts into a temporal framework. This is at least partially due to the fact that the bio-events recorded in organic-walled dinoflagellate cysts (dinocysts), which often represent the only microfossil group preserved, have not yet been calibrated to the international timescale. Here we present dinocyst ranges from Oligocene–Miocene sediments drilled offshore the Wilkes Land continental margin, East Antarctica (Integrated Ocean Drilling Program (IODP) Hole U1356A). In addition, we apply statistical means to test a priori assumptions about whether the recorded taxa were deposited in situ or were reworked from older strata. Moreover, we describe two new dinocyst species, Selenopemphix brinkhuisii sp. nov. and Lejeunecysta adeliensis sp. nov., which are identified as important markers for regional stratigraphic analysis. Finally, we calibrate all identified dinocyst events to the international timescale using independent age control from calcareous nanoplankton and magnetostratigraphy from IODP Hole U1356A, and we propose a provisional dinoflagellate cyst zonation scheme for the Oligocene–Miocene of the Southern Ocean.

scholarly journals Uncertainties in the modelled CO<sub>2</sub> threshold for Antarctic glaciation

2014 ◽  
Vol 10 (2) ◽  
pp. 451-466 ◽  
Author(s):  
E. Gasson ◽  
D. J. Lunt ◽  
R. DeConto ◽  
A. Goldner ◽  
M. Heinemann ◽  
...  
Keyword(s):  
Boundary Conditions ◽  
Climate Models ◽  
Climate Model ◽  
Ice Sheet ◽  
Atmospheric Co2 ◽  
Lapse Rate ◽  
Proxy Records ◽  
Co2 Concentrations ◽  
Model Dependency ◽  
Atmospheric Co2 Concentrations

Abstract. A frequently cited atmospheric CO2 threshold for the onset of Antarctic glaciation of ~780 ppmv is based on the study of DeConto and Pollard (2003) using an ice sheet model and the GENESIS climate model. Proxy records suggest that atmospheric CO2 concentrations passed through this threshold across the Eocene–Oligocene transition ~34 Ma. However, atmospheric CO2 concentrations may have been close to this threshold earlier than this transition, which is used by some to suggest the possibility of Antarctic ice sheets during the Eocene. Here we investigate the climate model dependency of the threshold for Antarctic glaciation by performing offline ice sheet model simulations using the climate from 7 different climate models with Eocene boundary conditions (HadCM3L, CCSM3, CESM1.0, GENESIS, FAMOUS, ECHAM5 and GISS_ER). These climate simulations are sourced from a number of independent studies, and as such the boundary conditions, which are poorly constrained during the Eocene, are not identical between simulations. The results of this study suggest that the atmospheric CO2 threshold for Antarctic glaciation is highly dependent on the climate model used and the climate model configuration. A large discrepancy between the climate model and ice sheet model grids for some simulations leads to a strong sensitivity to the lapse rate parameter.

scholarly journals Pan–ice-sheet glacier terminus change in East Antarctica reveals sensitivity of Wilkes Land to sea-ice changes

2016 ◽  
Vol 2 (5) ◽  
pp. e1501350 ◽  
Author(s):  
Bertie W. J. Miles ◽  
Chris R. Stokes ◽  
Stewart S. R. Jamieson
Keyword(s):  
Sea Ice ◽  
Drainage Basin ◽  
Ice Sheet ◽  
East Antarctica ◽  
Position Change ◽  
Outlet Glacier ◽  
Ocean Stratification ◽  
Glacier Terminus ◽  
Warm Deep Water ◽  
Wilkes Land

The dynamics of ocean-terminating outlet glaciers are an important component of ice-sheet mass balance. Using satellite imagery for the past 40 years, we compile an approximately decadal record of outlet-glacier terminus position change around the entire East Antarctic Ice Sheet (EAIS) marine margin. We find that most outlet glaciers retreated during the period 1974–1990, before switching to advance in every drainage basin during the two most recent periods, 1990–2000 and 2000–2012. The only exception to this trend was in Wilkes Land, where the majority of glaciers (74%) retreated between 2000 and 2012. We hypothesize that this anomalous retreat is linked to a reduction in sea ice and associated impacts on ocean stratification, which increases the incursion of warm deep water toward glacier termini. Because Wilkes Land overlies a large marine basin, it raises the possibility of a future sea level contribution from this sector of East Antarctica.

scholarly journals Late Oligocene obliquity-paced contourite sedimentation in the Wilkes Land margin of East Antarctica: implications for paleoceanographic and ice sheet configurations

2017 ◽  
Author(s):  
Ariadna Salabarnada ◽  
Carlota Escutia ◽  
Ursula Röhl ◽  
C. Hans Nelson ◽  
Robert McKay ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Ross Sea ◽  
Global Climate ◽  
Ice Sheet ◽  
East Antarctica ◽  
Late Oligocene ◽  
Early Oligocene ◽  
Geochemical Parameters ◽  
Wilkes Land

Abstract. The late Oligocene experienced atmospheric concentrations of CO2 between 400 and 750 ppm, which are within the IPCC projections for this century, assuming unabated CO2 emissions. However, Antarctic ice sheet and Southern Ocean paleoceanographic configurations during the late Oligocene are not well resolved, but are important to understand the influence of high-latitude Southern Hemisphere feedbacks on global climate under such CO2 scenarios. Here, we present late Oligocene (26–25 Ma) ice sheet and paleoceanographic reconstructions recorded in sediments recovered by IODP Site U1356, offshore of the Wilkes Land margin in East Antarctica. Our study, based on a combination of sediment facies analysis, physical properties, and geochemical parameters, shows that glacial and interglacial sediments are continuously reworked by bottom-currents, with maximum velocities occurring during the interglacial periods. Glacial sediments record poorly ventilated, low-oxygenation bottom water conditions, interpreted to represent a northward shift of westerly winds and surface oceanic fronts. During interglacial times, more oxygenated and ventilated conditions prevailed, which suggests enhanced mixing of the water masses with enhanced current velocities. Micritic limestone intervals within some of the interglacial facies represent warmer paleoclimatic conditions when less corrosive warmer northern component water (e.g. North Atlantic sourced deep water) had a greater influence on the site. The lack of iceberg rafted debris (IRD) throughout the studied interval contrasts with early Oligocene and post-Oligocene sections from Site U1356 and with late Oligocene strata from the Ross Sea (CRP and DSDP 270), which contain IRD and evidence for coastal sea ice and glaciers. These observations, supported by elevated paleotemperatures and the absence of sea-ice, suggest that between 26 and 25 Ma reduced glaciers or ice caps occupied the terrestrial lowlands of the Wilkes Land margin. Unlike today, the continental shelf was not over-deepened, and thus marine-based ice sheet expansion was likely limited to coastal regions. Combined, these data suggest that ice sheets in the Wilkes Subglacial Basin were largely land-based, and therefore retreated as a consequence of surface melt during late Oligocene, rather than direct ocean forcing and marine ice sheet instability processes as it did in younger past warm intervals. Spectral analysis on late Oligocene sediments from the eastern Wilkes Land margin show that the glacial-interglacial cyclicity and resulting displacements of the Southern Ocean frontal systems between 26–25 Ma were forced by obliquity.

scholarly journals Heterogenous CO<sub>2</sub> and CH<sub>4</sub> content of glacial meltwater of the Greenland Ice Sheet and implications for subglacial carbon processes

2020 ◽  
Author(s):  
Andrea J. Pain ◽  
Jonathan B. Martin ◽  
Ellen E. Martin ◽  
Shaily Rahman
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Mineral Weathering ◽  
Atmospheric Co2 ◽  
The Arctic ◽  
Co2 Concentrations ◽  
Geochemical Models ◽  
Carbon Dioxide Co2 ◽  
Atmospheric Co2 Concentrations ◽  
The Impact

Abstract. Accelerated melting of the Greenland Ice Sheet (GrIS) has increased freshwater delivery to the Arctic Ocean and amplified the need to understand the impact of GrIS meltwater on Arctic greenhouse gas (GHG) budgets. We measured carbon dioxide (CO2) and methane (CH4) concentrations and δ13C values and use geochemical models to evaluate subglacial CH4 and CO2 sources and sinks in water discharging from three subglacial outlets of the GrIS in southwest (Isunnguata and Russell Glaciers) and southern Greenland (Kiattut Sermiat). CH4 concentrations vary by orders of magnitude between sites and are saturated with respect to atmospheric concentrations at Kiattut Sermiat, but are supersaturated at southwest sites, even though oxidation reduces concentrations by up to 50 % during periods of low discharge. CO2 concentrations range from supersaturated at Isunnguata to undersaturated at Kiattut Sermiat. CO2 is consumed by mineral weathering throughout the melt season at all sites, however differences in the magnitude of subglacial CO2 sources result in meltwaters that are either sources or sinks of atmospheric CO2. The predominant source of CO2 at Isunnguata is organic matter (OM) remineralization, but Russell and Kiattut Sermiat sites have multiple or heterogeneous subglacial CO2 sources that maintain atmospheric CO2 concentrations at Russell but not at Kiattut Sermiat where CO2 is undersaturated. These results highlight the variability in GHG dynamics under the GrIS. Constraining this variability will improve our understanding of the impact of GrIS melt on Arctic GHG budgets, as well as the role of continental ice sheets in GHG variations over glacial-interglacial timescales.

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