scholarly journals A new mechanism for the two-step δ<sup>18</sup>O signal at the Eocene-Oligocene boundary

2011 ◽  
Vol 7 (1) ◽  
pp. 235-247 ◽  
Author(s):  
M. Tigchelaar ◽  
A. S. von der Heydt ◽  
H. A. Dijkstra
Keyword(s):  
Ocean Circulation ◽  
Atmospheric Carbon Dioxide ◽  
Global Climate ◽  
Carbon Dioxide Concentration ◽  
Threshold Value ◽  
Meridional Overturning Circulation ◽  
Global Changes ◽  
Modelling Studies ◽  
Antarctic Continent ◽  
The Antarctic

Abstract. The most marked step in the global climate transition from "Greenhouse" to "Icehouse" Earth occurred at the Eocene-Oligocene (E-O) boundary, 33.7 Ma. Evidence for climatic changes comes from many sources, including the marine benthic δ18O record, showing an increase by 1.2–1.5‰ at this time. This positive excursion is characterised by two steps, separated by a plateau. The increase in δ18O values has been attributed to rapid glaciation of the Antarctic continent, previously ice-free. Simultaneous changes in the δ13C record are suggestive of a greenhouse gas control on climate. Previous modelling studies show that a decline in pCO2 beyond a certain threshold value may have initiated the growth of a Southern Hemispheric ice sheet. These studies were not able to conclusively explain the remarkable two-step profile in δ18O. Furthermore, they considered changes in the ocean circulation only regionally, or indirectly through the oceanic heat transport. The potential role of global changes in ocean circulation in the E-O transition has not been addressed yet. Here a new interpretation of the δ18O signal is presented, based on model simulations using a simple coupled 8-box-ocean, 4-box-atmosphere model with an added land ice component. The model was forced with a slowly decreasing atmospheric carbon dioxide concentration. It is argued that the first step in the δ18O record reflects a shift in meridional overturning circulation from a Southern Ocean to a bipolar source of deep-water formation, which is associated with a cooling of the deep sea. The second step in the δ18O profile occurs due to a rapid glaciation of the Antarctic continent. This new mechanism is a robust outcome of our model and is qualitatively in close agreement with proxy data.

scholarly journals A new interpretation of the two-step <i>δ</i><sup>18</sup>O signal at the Eocene-Oligocene boundary

2010 ◽  
Vol 6 (4) ◽  
pp. 1391-1419 ◽  
Author(s):  
M. Tigchelaar ◽  
A. S. von der Heydt ◽  
H. A. Dijkstra
Keyword(s):  
Ocean Circulation ◽  
Atmospheric Carbon Dioxide ◽  
Global Climate ◽  
Carbon Dioxide Concentration ◽  
Density Perturbation ◽  
Threshold Value ◽  
Meridional Overturning Circulation ◽  
Antarctic Continent ◽  
New Interpretation ◽  
The Antarctic

Abstract. The most marked step in the global climate transition from "Greenhouse" to "Icehouse" Earth occurred at the Eocene-Oligocene (E–O) boundary, 33.7 Ma. Evidence for climatic changes comes from many sources, including the marine benthic δ18O record, showing an increase by 1.2–1.5‰ at this time. This positive excursion is characterised by two steps, separated by a plateau. The increase in δ18O values has been attributed to rapid glaciation of the Antarctic continent, previously ice-free. Simultaneous changes in the δ13C record are indicative of a greenhouse gas control on climate. Previous studies show that a decline in pCO2 beyond a certain threshold value may have initiated the growth of a Southern Hemispheric ice sheet. These studies were not able to conclusively explain the remarkable two-step profile in δ18O. Furthermore, they did not address the potential role of changes in ocean circulation in the E–O transition. Here a new interpretation of the δ18O signal is presented, based on model simulations using a simple coupled 8-box-ocean, 4-box-atmosphere model with an added land ice component. The model was forced with a slowly decreasing atmospheric carbon dioxide concentration. It is argued that the first step in the δ18O represents a shift in meridional overturning circulation from a Southern Ocean to a bipolar source of deep-water formation, which is associated with a cooling of the deep sea. This shift can be initiated by a small density perturbation in the model, although there is also a parameter regime for which the shift occurs spontaneously. The second step in the δ18O profile occurs due to a rapid glaciation of the Antarctic continent. This new interpretation is a robust outcome of our model and is in good agreement with proxy data.

Global Ocean Meridional Overturning

2007 ◽  
Vol 37 (10) ◽  
pp. 2550-2562 ◽  
Author(s):  
Rick Lumpkin ◽  
Kevin Speer
Keyword(s):  
Ocean Circulation ◽  
Global Scale ◽  
The Arctic ◽  
Global Ocean ◽  
Mixing Processes ◽  
Boundary Currents ◽  
Antarctic Continent ◽  
Meridional Overturning ◽  
Global Ocean Circulation ◽  
The Antarctic

Abstract A decade-mean global ocean circulation is estimated using inverse techniques, incorporating air–sea fluxes of heat and freshwater, recent hydrographic sections, and direct current measurements. This information is used to determine mass, heat, freshwater, and other chemical transports, and to constrain boundary currents and dense overflows. The 18 boxes defined by these sections are divided into 45 isopycnal (neutral density) layers. Diapycnal transfers within the boxes are allowed, representing advective fluxes and mixing processes. Air–sea fluxes at the surface produce transfers between outcropping layers. The model obtains a global overturning circulation consistent with the various observations, revealing two global-scale meridional circulation cells: an upper cell, with sinking in the Arctic and subarctic regions and upwelling in the Southern Ocean, and a lower cell, with sinking around the Antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges.

scholarly journals Using New Gas Exchange Methods to Estimate Mesophyll Conductance and Non-stomatal Inhibition of Photosynthesis Caused by Water Deficits

HortScience ◽  
2012 ◽  
Vol 47 (6) ◽  
pp. 687-690 ◽  
Author(s):  
James Bunce
Keyword(s):  
Carbon Dioxide ◽  
Water Stress ◽  
Soil Water ◽  
Atmospheric Carbon Dioxide ◽  
Global Climate ◽  
Stomatal Closure ◽  
Carbon Dioxide Concentration ◽  
Mesophyll Conductance ◽  
Water Deficits ◽  
Air Space

Soil water deficits remain one of the most important factors reducing the yield of crop plants and may become even more limiting with changes in the global climate and competition for fresh water resources. Soil water deficits reduce plant growth partly by reducing photosynthesis. However, it remains unclear how important non-stomatal factors are in limiting photosynthesis under moderate water stress and whether rising atmospheric carbon dioxide may alter which processes limit photosynthesis under water stress. The conductance to CO2 from the substomatal air space to the site of carboxylation inside chloroplasts in C3 plants is now termed mesophyll conductance. Because of the competition between CO2 and O2 for RuBisco, the carbon dioxide concentration at the chloroplast can be estimated from the O2 sensitivity of photosynthesis, providing a new method of estimating mesophyll conductance. It has also recently been realized that partial stomatal closure resulting from water stress can often be reversed by exposing leaves to low CO2. This provides a new means of assessing the non-stomatal component of the inhibition of photosynthesis by water stress. These methods were applied to four C3 species and revealed that mesophyll conductance decreased substantially with water stress in two of the four species and that reopening of stomata did not eliminate the reduction in photosynthesis caused by moderate water stress at either the current ambient or elevated CO2 concentrations.

scholarly journals Larger Role for Shallow Intermediate Waters in Ocean Circulation

Eos ◽  
2020 ◽  
Vol 101 ◽  
Author(s):  
Sara Pratt
Keyword(s):  
Ocean Circulation ◽  
Global Climate ◽  
Atlantic Meridional Overturning Circulation ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Intermediate Waters

Water masses formed off southeastern Greenland may contribute more than previously thought to the variability of the Atlantic Meridional Overturning Circulation, which strongly influences global climate.

Preliminary biostratigraphy of IODP Expedition 383 sites

2021 ◽  
Author(s):  
Mariem Saavedra-Pellitero ◽  
Anieke Brombacher ◽  
Oliver Esper ◽  
Alexandre de Souza ◽  
Elisa Malinverno ◽  
...  
Keyword(s):  
High Resolution ◽  
Water Mass ◽  
Global Climate ◽  
Drake Passage ◽  
Meridional Overturning Circulation ◽  
The Pacific ◽  
Meridional Overturning ◽  
Circumpolar Current ◽  
Oceanic Carbon ◽  
The Antarctic

&lt;p&gt;The Antarctic Circumpolar Current (ACC) is a major driver of global climate. It connects all three ocean basins, integrating global climate variability, and its vertical water mass structure plays a key role in oceanic carbon storage. The Atlantic and Indian sectors of the ACC are well studied, but the Pacific sector lacks deep-sea drilling records. Therefore, past water mass transport through the Drake Passage and its effect on Atlantic Meridional Overturning Circulation are not well understood. To fill this gap, IODP Expedition 383 recovered sediments from three sites in the central South Pacific and three sites from the southern Chilean Margin.&lt;/p&gt;&lt;p&gt;Here we present the preliminary biostratigraphy developed during the expedition. The sediments contained abundant nannofossils, foraminifera, radiolarians, diatoms and silicoflagellates which produced age models that were in excellent agreement with the shipboard magnetostratigraphy. Two sites contain high-resolution Pleistocene records, one site goes back to the Pliocene, and two others reach back to the late Miocene. Post-cruise research will further refine these age models through high-resolution bio-, magneto- and oxygen isotope stratigraphies that are currently being generated.&lt;/p&gt;

Quantitative precipitation estimation in Antarctica using different ZE-SR relationships based on snowfall classification combining ground observations by radar and disdrometer

2020 ◽  
Author(s):  
Alessandro Bracci ◽  
Nicoletta Roberto ◽  
Luca Baldini ◽  
Mario Montopoli ◽  
Elisa Adirosi ◽  
...  
Keyword(s):  
Quantitative Estimate ◽  
Global Climate ◽  
Ice Sheet ◽  
Estimation Methods ◽  
Antarctic Ice Sheet ◽  
Numerical Weather ◽  
Precipitation Estimation ◽  
Antarctic Continent ◽  
The Antarctic ◽  
Reflectivity Factor

&lt;p&gt;The Antarctic Ice Sheet plays a major role in regional and global climate variability and represents, probably, the most critical factor of future sea-level rise. Snow and solid precipitation more broadly have been recognized as primary mass input for ice sheet. However, despite its fundamental role in the surface mass balance estimation, precipitation over Polar region and in the Antarctica particularly, remains largely unknown, being not well assessed by numerical weather/climate models, by ground observations and satellite measurements as well. More accurate estimations of precipitation in the Antarctic continent are desirable not only in understanding the behavior of the Antarctic Ice Sheet, but also in validating global climate and numerical weather prediction models and also in order to constrain measurements from space during validation/calibration satellite campaigns.&lt;/p&gt;&lt;p&gt;Recently, several observatories in Antarctica have been equipped with equipment for cloud and precipitation measurements, such as the two Italian stations &amp;#8220;Mario Zucchelli&amp;#8221;, Terra Nova Bay, and Concordia, in the Antarctic Plateau. At &amp;#8220;Mario Zucchelli&amp;#8221;, instrumentation includes 24-GHz vertical pointing radar Micro Rain Radar (MRR) and optical disdrometer. The synergetic use of such set of instruments allows for characterizing and quantifying precipitation, even if quantitative estimate of precipitation from radar is extremely demanding, especially in snowfall, because of variability microphysical features of hydrometeors.&lt;/p&gt;&lt;p&gt;Usually precipitation estimation methods with weather radar are based on relationships between radar equivalent reflectivity factor (Ze) and liquid equivalent snowfall rate (SR). Several relationships are reported in literature, derived from comparison between radar and ground sensors but very few are suitable for the Antarctic continent and none also considers the microphysical characterization of hydrometeors.&lt;/p&gt;&lt;p&gt;This work shows quantitative estimate of the Antarctic precipitation for several snow episodes at the Mario Zucchelli station using specific ZE-SR relationships also taking into account the snowfall classification according to dominating hydrometeor type (e.g. pristine, aggregate, dendrite, plate). Microphysical properties of precipitation are inferred by comparing radar measurements with simulations obtained from disdrometer measurements in terms of reflectivity factor. Specifically, the Ze directly derived by radar has been compared with the Ze calculated by disdrometer observations coupling particle size distributions and NASA database of hydrometeor backscattering values based on the Discrete Dipole Approximation. More challenging are estimations at Concordia, where ice particles have very small sizes and are hardly detectable by laser disdrometer, and where MRR lacks of adequate sensitivity.&lt;/p&gt;

Southern Ocean upwelling, Earth’s obliquity, and glacial-interglacial atmospheric CO2 change

Science ◽  
2020 ◽  
Vol 370 (6522) ◽  
pp. 1348-1352
Author(s):  
Xuyuan E. Ai ◽  
Anja S. Studer ◽  
Daniel M. Sigman ◽  
Alfredo Martínez-García ◽  
François Fripiat ◽  
...  
Keyword(s):  
Carbon Dioxide ◽  
Late Pleistocene ◽  
Atmospheric Carbon Dioxide ◽  
Global Climate ◽  
Nitrogen Isotope ◽  
Ice Ages ◽  
Glacial Cycles ◽  
Atmospheric Carbon ◽  
Carbon Dioxide Levels ◽  
The Antarctic

Previous studies have suggested that during the late Pleistocene ice ages, surface-deep exchange was somehow weakened in the Southern Ocean’s Antarctic Zone, which reduced the leakage of deeply sequestered carbon dioxide and thus contributed to the lower atmospheric carbon dioxide levels of the ice ages. Here, high-resolution diatom-bound nitrogen isotope measurements from the Indian sector of the Antarctic Zone reveal three modes of change in Southern Westerly Wind–driven upwelling, each affecting atmospheric carbon dioxide. Two modes, related to global climate and the bipolar seesaw, have been proposed previously. The third mode—which arises from the meridional temperature gradient as affected by Earth’s obliquity (axial tilt)—can explain the lag of atmospheric carbon dioxide behind climate during glacial inception and deglaciation. This obliquity-induced lag, in turn, makes carbon dioxide a delayed climate amplifier in the late Pleistocene glacial cycles.

scholarly journals The Global Climate Response to Lowering Surface Orography of Antarctica and the Importance of Atmosphere–Ocean Coupling

2016 ◽  
Vol 29 (11) ◽  
pp. 4137-4153 ◽  
Author(s):  
Hansi K. A. Singh ◽  
Cecilia M. Bitz ◽  
Dargan M. W. Frierson
Keyword(s):  
Energy Transport ◽  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
Longwave Radiation ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
The Arctic ◽  
System Response ◽  
The Antarctic

Abstract A global climate model is used to study the effect of flattening the orography of the Antarctic Ice Sheet on climate. A general result is that the Antarctic continent and the atmosphere aloft warm, while there is modest cooling globally. The large local warming over Antarctica leads to increased outgoing longwave radiation, which drives anomalous southward energy transport toward the continent and cooling elsewhere. Atmosphere and ocean both anomalously transport energy southward in the Southern Hemisphere. Near Antarctica, poleward energy and momentum transport by baroclinic eddies strengthens. Anomalous southward cross-equatorial energy transport is associated with a northward shift in the intertropical convergence zone. In the ocean, anomalous southward energy transport arises from a slowdown of the upper cell of the oceanic meridional overturning circulation and a weakening of the horizontal ocean gyres, causing sea ice in the Northern Hemisphere to expand and the Arctic to cool. Comparison with a slab-ocean simulation confirms the importance of ocean dynamics in determining the climate system response to Antarctic orography. This paper concludes by briefly presenting a discussion of the relevance of these results to climates of the past and to future climate scenarios.

scholarly journals Pending recovery in the strength of the meridional overturning circulation at 26° N

2020 ◽  
Author(s):  
Ben I. Moat ◽  
David A. Smeed ◽  
Eleanor Frajka-Williams ◽  
Damien G. Desbruyères ◽  
Claudie Beaulieu ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Large Scale ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Ocean Reanalysis ◽  
Buoyancy Forcing ◽  
Modelling Studies ◽  
Meridional Overturning ◽  
Lower Magnitude

Abstract. The strength of the Atlantic meridional overturning circulation (AMOC) at 26° N has now been continuously measured by the RAPID array over the period Apr 2004–Sept 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transports from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind-forcing, contrasting with previous expectations about a slowly-varying, buoyancy forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic Multidecadal Variability (AMV) which has been steadily declining since a peak in 2008–2010. In 2013–2015, a period of strong buoyancy-forcing by the atmosphere drove intense watermass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing. Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3–9 years. At 45° N, observations suggest that the AMOC my already be increasing. We have therefore examined the record of transports at 26° N to see whether the AMOC in the subtropical North Atlantic is now recovering from a previously reported low period commencing in 2009. Comparing the two latitudes, the AMOC at 26° N is higher than its previous low. Extending the record at 26° N with ocean reanalysis from GloSea5, the transport fluctuations follow those at 45° N by 0–2 years, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering.

Contrasting sources of variability in subtropical and subpolar Atlantic overturning

2020 ◽  
Author(s):  
Yavor Kostov ◽  
Helen L. Johnson ◽  
David P. Marshall ◽  
Gael Forget ◽  
Patrick Heimbach ◽  
...  
Keyword(s):  
Wind Stress ◽  
Ocean Circulation ◽  
Global Climate ◽  
Surface Wind ◽  
Circulation Model ◽  
Ocean Surface ◽  
Meridional Overturning Circulation ◽  
Surface Wind Stress ◽  
History Of ◽  
Meridional Overturning

&lt;p&gt;&lt;strong&gt;The Atlantic meridional overturning circulation (AMOC) is pivotal for regional and global climate due to its key role in the uptake and redistribution of heat, carbon and other tracers. Establishing the causes of historical variability in the AMOC can tell us how the circulation responds to natural and anthropogenic changes at the ocean surface. However, attributing observed AMOC variability and inferring causal relationships is challenging because the circulation is influenced by multiple factors which co-vary and whose overlapping impacts can persist for years.&amp;#160; Here we reconstruct and unambiguously attribute variability in the AMOC at the latitudes of two observational arrays to the recent history of surface wind stress, temperature and salinity. We use a state-of-the-art technique that computes space- and time-varying sensitivity patterns of the AMOC strength with respect to multiple surface properties from a numerical ocean circulation model constrained by observations. While on inter-annual timescales, AMOC variability at 26&amp;#176;N is overwhelmingly dominated by a linear response to local wind stress, in contrast, AMOC variability at subpolar latitudes is generated by both wind stress and surface temperature and salinity anomalies. Our analysis allows us to obtain the first-ever reconstruction of subpolar AMOC from forcing anomalies at the ocean surface.&lt;/strong&gt;&lt;/p&gt;

Sign in / Sign up

Export Citation Format

Share Document