Supplementary material to "Southern Ocean bottom water cooling and ice sheet expansion during the middle Miocene climate transition"

2020 ◽  
Author(s):  
Thomas J. Leutert ◽  
Sevasti Modestou ◽  
Stefano M. Bernasconi ◽  
A. Nele Meckler
Keyword(s):  
Southern Ocean ◽  
Bottom Water ◽  
Middle Miocene ◽  
Ice Sheet ◽  
Ocean Bottom ◽  
Water Cooling ◽  
Supplementary Material ◽  
Climate Transition

scholarly journals Southern Ocean bottom water cooling and ice sheet expansion during the middle Miocene climate transition

2020 ◽  
Author(s):  
Thomas J. Leutert ◽  
Sevasti Modestou ◽  
Stefano M. Bernasconi ◽  
A. Nele Meckler
Keyword(s):  
Southern Ocean ◽  
Bottom Water ◽  
Middle Miocene ◽  
Ocean Bottom ◽  
Ocean Drilling Program ◽  
Bottom Water Temperature ◽  
Ice Volume ◽  
Global Ice Volume ◽  
Clumped Isotope ◽  
Climate Transition

Abstract. The middle Miocene climate transition (MMCT, ~14.5–13.0 Ma) was associated with a significant expansion of Antarctic ice, but the mechanisms triggering the event remain enigmatic. We present a new clumped isotope (∆47) bottom water temperature (BWT) record from 16.0 Ma to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean, and compare it to existing BWT records. We show that BWTs in the Southern Ocean were ~8–10 °C during the middle Miocene greenhouse, and thus considerably warmer than today. Nonetheless, bottom water δ18O (calculated from foraminiferal δ18O and ∆47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset demonstrates that BWTs at Site 747 decreased by ~3–5 °C across the MMCT. This cooling preceded the stepped main increase in global ice volume, and appears to have been followed by a transient bottom water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Additional processes and feedbacks required for substantial ice growth may have contributed to the observed decoupling of Southern Ocean BWT and global ice volume.

scholarly journals Southern Ocean bottom-water cooling and ice sheet expansion during the middle Miocene climate transition

2021 ◽  
Vol 17 (5) ◽  
pp. 2255-2271
Author(s):  
Thomas J. Leutert ◽  
Sevasti Modestou ◽  
Stefano M. Bernasconi ◽  
A. Nele Meckler
Keyword(s):  
Southern Ocean ◽  
Bottom Water ◽  
Middle Miocene ◽  
Ocean Bottom ◽  
Ocean Drilling Program ◽  
Bottom Water Temperature ◽  
Ice Volume ◽  
Global Ice Volume ◽  
Clumped Isotope ◽  
Climate Transition

Abstract. The middle Miocene climate transition (MMCT), around 14 Ma, was associated with a significant climatic shift, but the mechanisms triggering the event remain enigmatic. We present a clumped isotope (Δ47) bottom-water temperature (BWT) record from 16.0 to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean and compare it to existing BWT records from different latitudes. We show that BWTs in the Southern Ocean reached 8–10 ∘C during the Miocene climatic optimum. These high BWT values indicate considerably warmer bottom-water conditions than today. Nonetheless, bottom-water δ18O (calculated from foraminiferal δ18O and Δ47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset further demonstrates that BWTs at Site 747 were variable with an overall cooling trend across the MMCT. Notably, a cooling of around 3–5 ∘C preceded the stepped main increase in benthic δ18O, interpreted as global ice volume expansion, and appears to have been followed by a transient bottom-water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom-water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Feedbacks required for substantial ice growth and/or tectonic processes may have contributed to the observed decoupling of global ice volume and Southern Ocean BWT.

scholarly journals Postglacial response of Arctic Ocean gas hydrates to climatic amelioration

2017 ◽  
Vol 114 (24) ◽  
pp. 6215-6220 ◽  
Author(s):  
Pavel Serov ◽  
Sunil Vadakkepuliyambatta ◽  
Jürgen Mienert ◽  
Henry Patton ◽  
Alexey Portnov ◽  
...  
Keyword(s):  
Arctic Ocean ◽  
Gas Hydrates ◽  
Gas Hydrate ◽  
Bottom Water ◽  
Environmental Changes ◽  
Thermal Dissociation ◽  
Ice Sheet ◽  
The Arctic ◽  
Ocean Bottom ◽  
Methane Release

Seafloor methane release due to the thermal dissociation of gas hydrates is pervasive across the continental margins of the Arctic Ocean. Furthermore, there is increasing awareness that shallow hydrate-related methane seeps have appeared due to enhanced warming of Arctic Ocean bottom water during the last century. Although it has been argued that a gas hydrate gun could trigger abrupt climate change, the processes and rates of subsurface/atmospheric natural gas exchange remain uncertain. Here we investigate the dynamics between gas hydrate stability and environmental changes from the height of the last glaciation through to the present day. Using geophysical observations from offshore Svalbard to constrain a coupled ice sheet/gas hydrate model, we identify distinct phases of subglacial methane sequestration and subsequent release on ice sheet retreat that led to the formation of a suite of seafloor domes. Reconstructing the evolution of this dome field, we find that incursions of warm Atlantic bottom water forced rapid gas hydrate dissociation and enhanced methane emissions during the penultimate Heinrich event, the Bølling and Allerød interstadials, and the Holocene optimum. Our results highlight the complex interplay between the cryosphere, geosphere, and atmosphere over the last 30,000 y that led to extensive changes in subseafloor carbon storage that forced distinct episodes of methane release due to natural climate variability well before recent anthropogenic warming.

scholarly journals What Fraction of the Pacific and Indian Oceans' Deep Water is formed in the North Atlantic?

2018 ◽  
Author(s):  
James W. B. Rae ◽  
Wally Broecker
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Deep Water ◽  
Bottom Water ◽  
Ocean Bottom ◽  
The North ◽  
Water Value ◽  
The Pacific ◽  
Northern Atlantic ◽  
The North Atlantic

Abstract. In this contribution we explore constraints on the fractions of deep water present in Indian and Pacific Oceans which originated in the northern Atlantic and in the Southern Ocean. Based on PO4* we show that if ventilated Antarctic shelf waters characterize the Southern contribution, then the proportions are close to 50–50. If instead a Southern Ocean bottom water value is used, the Southern contribution is increased to 75 %. While this larger estimate may characterize the volume of water entering the Indo-Pacific from the Southern Ocean, it contains a significant portion of entrained northern water. We also note that ventilation may be highly tracer dependent: for instance Southern Ocean waters may contribute only 35 % of the deep radiocarbon budget, even if their volumetric contribution is 75 %. In our estimation, the most promising approaches involve using CFC-11 to constrain the amount of deep water formed in the Southern Ocean.

scholarly journals What fraction of the Pacific and Indian oceans' deep water is formed in the Southern Ocean?

2018 ◽  
Vol 15 (12) ◽  
pp. 3779-3794 ◽  
Author(s):  
James W. B. Rae ◽  
Wally Broecker
Keyword(s):  
Southern Ocean ◽  
Deep Water ◽  
Large Scale ◽  
Bottom Water ◽  
Deep Ocean ◽  
Ocean Bottom ◽  
Biological Pump ◽  
Water Value ◽  
The Pacific ◽  
Northern Atlantic

Abstract. In this contribution we explore constraints on the fractions of deep water present in the Indian and Pacific oceans which originated in the northern Atlantic and in the Southern Ocean. Based on PO4* we show that if ventilated Antarctic shelf waters characterize the Southern contribution, then the proportions could be close to 50–50. If instead a Southern Ocean bottom water value is used, the Southern contribution is increased to 75 %. While this larger estimate may best characterize the volume of water entering the Indo-Pacific from the Southern Ocean, it contains a significant portion of entrained northern water. We also note that ventilation may be highly tracer dependent: for instance Southern Ocean waters may contribute only 35 % of the deep radiocarbon budget, even if their volumetric contribution is 75 %. In our estimation, the most promising approaches involve using CFC-11 to constrain the amount of deep water formed in the Southern Ocean. Finally, we highlight the broad utility of PO4* as a tracer of deep water masses, including descending plumes of Antarctic Bottom Water and large-scale patterns of deep ocean mixing, and as a tracer of the efficiency of the biological pump.

scholarly journals Paleoceanography and ice sheet variability offshore Wilkes Land, Antarctica – Part 1: Insights from late Oligocene astronomically paced contourite sedimentation

2018 ◽  
Vol 14 (7) ◽  
pp. 991-1014 ◽  
Author(s):  
Ariadna Salabarnada ◽  
Carlota Escutia ◽  
Ursula Röhl ◽  
C. Hans Nelson ◽  
Robert McKay ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Bottom Water ◽  
Global Climate ◽  
Ice Sheet ◽  
Polar Front ◽  
Geochemical Data ◽  
Late Oligocene ◽  
Water Conditions ◽  
Wilkes Land

Abstract. Antarctic ice sheet and Southern Ocean paleoceanographic configurations during the late Oligocene are not well resolved. They are however important to understand the influence of high-latitude Southern Hemisphere feedbacks on global climate under CO2 scenarios (between 400 and 750 ppm) projected by the IPCC for this century, assuming unabated CO2 emissions. Sediments recovered by the Integrated Ocean Drilling Program (IODP) at Site U1356, offshore of the Wilkes Land margin in East Antarctica, provide an opportunity to study ice sheet and paleoceanographic configurations during the late Oligocene (26–25 Ma). Our study, based on a combination of sediment facies analysis, magnetic susceptibility, density, and X-ray fluorescence geochemical data, 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 as resulting from a northward shift of westerly winds and surface oceanic fronts. Interglacial sediments record more oxygenated and ventilated bottom water conditions and strong current velocities, which suggests enhanced mixing of the water masses as a result of a southward shift of the polar front. Intervals with preserved carbonated nannofossils within some of the interglacial facies are interpreted as forming under warmer paleoclimatic conditions when less corrosive warmer northern component water (e.g., North Atlantic sourced deep water) had a greater influence on the site. Spectral analysis on the late Oligocene sediment interval shows that the glacial–interglacial cyclicity and related displacements of the Southern Ocean frontal systems between 26 and 25 Ma were forced mainly by obliquity. The paucity of iceberg-rafted debris (IRD) throughout the studied interval contrasts with earlier Oligocene and post-Miocene Climate Optimum sections from Site U1356 and with late Oligocene strata from the Ross Sea, which contain IRD and evidence for coastal glaciers and sea ice. These observations, supported by elevated sea surface paleotemperatures, the absence of sea ice, and reconstructions of fossil pollen between 26 and 25 Ma at Site U1356, suggest that open-ocean water conditions prevailed. Combined, this evidence suggests that glaciers or ice caps likely occupied the topographic highs and lowlands of the now marine Wilkes Subglacial Basin (WSB). Unlike today, the continental shelf was not overdeepened and thus ice sheets in the WSB were likely land-based, and marine-based ice sheet expansion was likely limited to coastal regions.

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