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 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 Warm Middle Miocene Indian Ocean Bottom Water Temperatures: Comparison of Clumped Isotope and Mg/Ca‐Based Estimates

2020 ◽  
Vol 35 (11) ◽  
Author(s):  
S. E. Modestou ◽  
T. J. Leutert ◽  
A. Fernandez ◽  
C. H. Lear ◽  
A. N. Meckler
Keyword(s):  
Indian Ocean ◽  
Bottom Water ◽  
Middle Miocene ◽  
Ocean Bottom ◽  
Clumped Isotope

scholarly journals Boundary conditions for the Middle Miocene Climate Transition (MMCT v1.0)

2017 ◽  
Author(s):  
Amanda Frigola ◽  
Matthias Prange ◽  
Michael Schulz
Keyword(s):  
Boundary Conditions ◽  
General Circulation ◽  
Middle Miocene ◽  
Climate Models ◽  
Global Climate ◽  
Co2 Concentration ◽  
General Circulation Models ◽  
Global Climate Models ◽  
Ice Volume ◽  
Climate Transition

Abstract. The Middle Miocene Climate Transition was characterized by major Antarctic ice-sheet expansion and global cooling during the interval ~ 15–13 Ma. Here we present two sets of boundary conditions for global general circulation models characterizing the periods before (Middle Miocene Climatic Optimum; MMCO) and after (Middle Miocene Glaciation; MMG) the transition. These boundary conditions include Middle Miocene global topography, bathymetry and vegetation. Additionally, Antarctic ice volume and geometry, sea-level and atmospheric CO2 concentration estimates for the MMCO and the MMG are reviewed. The boundary-condition files are available for use as input in a wide variety of global climate models and constitute a valuable tool for modeling studies with a focus on the Middle Miocene.

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 Geothermal heating and episodic cold-seawater intrusions into an isolated ridge-flank basin near the Mid-Atlantic Ridge

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Keir Becker ◽  
Richard E. Thomson ◽  
Earl E. Davis ◽  
Heinrich Villinger ◽  
C. Geoffrey Wheat
Keyword(s):  
Bottom Water ◽  
Ocean Bottom ◽  
Geothermal Heat ◽  
Bottom Water Temperature ◽  
Geothermal Heating ◽  
Mid Ocean Ridge ◽  
Before And After ◽  
Bottom Waters ◽  
Mid Atlantic Ridge ◽  
Ocean Ridge

AbstractSix-year records of ocean bottom water temperatures at two locations in an isolated, sedimented deep-water (∼4500 m) basin on the western flank of the mid-Atlantic Ridge reveal long periods (months to >1 year) of slow temperature rises punctuated by more rapid (∼1 month) cooling events. The temperature rises are consistent with a combination of gradual heating by the geothermal flux through the basin and by diapycnal mixing, while the sharper cooling events indicate displacement of heated bottom waters by incursions of cold, dense bottom water over the deepest part of the sill bounding the basin. Profiles of bottom water temperature, salinity, and oxygen content collected just before and after a cooling event show a distinct change in the water mass suggestive of an incursion of diluted Antarctic Bottom Water from the west. Our results reveal details of a mechanism for the transfer of geothermal heat and bottom water renewal that may be common on mid-ocean ridge flanks.

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.

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