scholarly journals Contrasting late-glacial paleoceanographic evolution between the upper and lower continental slope of the western South Atlantic

2020 ◽  
Vol 16 (4) ◽  
pp. 1245-1261
Author(s):  
Leticia G. Luz ◽  
Thiago P. Santos ◽  
Timothy I. Eglinton ◽  
Daniel Montluçon ◽  
Blanca Ausin ◽  
...  
Keyword(s):  
Rio De Janeiro ◽  
Planktonic Foraminifera ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Coastal Current ◽  
Limited Information ◽  
Last Deglaciation ◽  
Glacial Cycle ◽  
Last Glacial ◽  
The Last Glacial

Abstract. The number of sedimentary records collected along the Brazilian continental margin has increased significantly in recent years, but relatively few are located in shallow waters and register paleoceanographic processes in the outer shelf–middle slope prior to 10–15 ka. For instance, the northward flow up to 23–24∘ S of cold and fresh shelf waters sourced from the Subantarctic region is an important feature of current hydrodynamics in the subtropical western South Atlantic Ocean, and yet limited information is available for the long-term changes of this system. Herein, we considered a suite of organic and inorganic proxies – alkenones-derived sea surface temperature (SST), δD-alkenones, δ18O of planktonic foraminifera, and ice-volume free seawater δ18OIVF−SW – in sediment from two cores (RJ-1501 and RJ-1502) collected off the Rio de Janeiro Shelf (SE Brazilian continental shelf) to shed light on SST patterns and relative salinity variations since the end of the last glacial cycle in the region and the implications of these processes over a broader spatial scale. The data indicate that, despite the proximity (∼40 km apart) of both cores, apparently contradictory climatic evolution occurred at the two sites, with the shallower (deeper) core RJ-1501 (RJ-1502) showing consistently cold (warm) and fresh (salt) conditions toward the Last Glacial Maximum (LGM) and last deglaciation. This can be reconciled by considering that the RJ-1501 core registered a signal from mid- to high latitudes on the upper slope off Rio de Janeiro represented by the influence of the cold and fresh waters composed of Subantarctic Shelf Water and La Plata Plume Water transported northward by the Brazilian Coastal Current (BCC). The data from core RJ-1502 and previous information for deep-cores from the same region support this interpretation. In addition, alkenone-derived SST and δ18OIVF−SW suggest a steep thermal and density gradient formed between the BCC and Brazil Current (BC) during the last climate transition which, in turn, may have generated perturbations in the air–sea heat flux with consequences for the regional climate of SE South America. In a scenario of future weakening of the Atlantic Meridional Overturning Circulation, the reconstructed gradient may become a prominent feature of the region.

scholarly journals <i>δ</i><sup>13</sup>C decreases in the upper western South Atlantic during Heinrich Stadials 3 and 2

2017 ◽  
Vol 13 (4) ◽  
pp. 345-358 ◽  
Author(s):  
Marília C. Campos ◽  
Cristiano M. Chiessi ◽  
Ines Voigt ◽  
Alberto R. Piola ◽  
Henning Kuhnert ◽  
...  
Keyword(s):  
Climate Change ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Glacial Period ◽  
Last Deglaciation ◽  
Last Glacial Period ◽  
Last Glacial ◽  
Change Events ◽  
The Last Glacial ◽  
Millennial Scale

Abstract. Abrupt millennial-scale climate change events of the last deglaciation (i.e. Heinrich Stadial 1 and the Younger Dryas) were accompanied by marked increases in atmospheric CO2 (CO2atm) and decreases in its stable carbon isotopic ratios (δ13C), i.e. δ13CO2atm, presumably due to outgassing from the ocean. However, information on the preceding Heinrich Stadials during the last glacial period is scarce. Here we present δ13C records from two species of planktonic foraminifera from the western South Atlantic that reveal major decreases (up to 1 ‰) during Heinrich Stadials 3 and 2. These δ13C decreases are most likely related to millennial-scale periods of weakening of the Atlantic meridional overturning circulation and the consequent increase (decrease) in CO2atm (δ13CO2atm). We hypothesise two mechanisms that could account for the decreases observed in our records, namely strengthening of Southern Ocean deep-water ventilation and weakening of the biological pump. Additionally, we suggest that air–sea gas exchange could have contributed to the observed δ13C decreases. Together with other lines of evidence, our data are consistent with the hypothesis that the CO2 added to the atmosphere during abrupt millennial-scale climate change events of the last glacial period also originated in the ocean and reached the atmosphere by outgassing. The temporal evolution of δ13C during Heinrich Stadials 3 and 2 in our records is characterized by two relative minima separated by a relative maximum. This w structure is also found in North Atlantic and South American records, further suggesting that such a structure is a pervasive feature of Heinrich Stadial 2 and, possibly, also Heinrich Stadial 3.

scholarly journals Rapid bottom-water circulation changes during the last glacial cycle in the coastal low-latitude NE Atlantic

2014 ◽  
Vol 81 (2) ◽  
pp. 330-338 ◽  
Author(s):  
David Gallego-Torres ◽  
Oscar E. Romero ◽  
Francisca Martínez-Ruiz ◽  
Jung-Hyun Kim ◽  
Barbara Donner ◽  
...  
Keyword(s):  
High Resolution ◽  
Deep Water ◽  
Bottom Water ◽  
Meridional Overturning Circulation ◽  
Last Deglaciation ◽  
Glacial Cycle ◽  
Last Glacial ◽  
Deep Waters ◽  
High Resolution Reconstruction ◽  
The Last Glacial

AbstractPrevious paleoceanographic studies along the NW African margin focused on the dynamics of surface and intermediate waters, whereas little attention has been devoted to deep-water masses. Currently, these deep waters consist mainly of North Atlantic Deep Waters as part of the Atlantic Meridional Overturning Circulation (AMOC). However, this configuration was altered during periods of AMOC collapse. We present a high-resolution reconstruction of bottom-water ventilation and current evolution off Mauritania from the last glacial maximum into the early Holocene. Applying redox proxies (Mo, U and Mn) measured on sediments from off Mauritania, we describe changes in deep-water oxygenation and we infer the evolution of deep-water conditions during millennial-scale climate/oceanographic events in the area. The second half of Heinrich Event 1 and the Younger Dryas were recognized as periods of reduced ventilation, coinciding with events of AMOC reduction. We propose that these weakening circulation events induced deficient deep-water oxygenation in the Mauritanian upwelling region, which together with increased productivity promoted reducing conditions and enhanced organic-matter preservation. This is the first time the effect of AMOC collapse in the area is described at high resolution, broadening the knowledge on basin-wide oceanographic changes associated with rapid climate variability during the last deglaciation.

Strong and deep Atlantic meridional overturning circulation during the last glacial cycle

Nature ◽  
2014 ◽  
Vol 517 (7532) ◽  
pp. 73-76 ◽  
Author(s):  
E. Böhm ◽  
J. Lippold ◽  
M. Gutjahr ◽  
M. Frank ◽  
P. Blaser ◽  
...  
Keyword(s):  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Glacial Cycle ◽  
Overturning Circulation ◽  
Last Glacial ◽  
Meridional Overturning ◽  
The Last Glacial

Orbital cycle-related benthic-pelagic fluctuations in Foraminifera during the last glacial-interglacial interval in the western South Atlantic

2021 ◽  
Author(s):  
Jaime Yesid Suarez Ibarra ◽  
Cristiane Fraga Frozza ◽  
Sandro Monticelli Petró ◽  
Pamela de Lara Palhano ◽  
Maria Alejandra Gómez Pivel
Keyword(s):  
Deep Water ◽  
Bottom Water ◽  
South Atlantic ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
Biological Pump ◽  
Sea Floor ◽  
Last Glacial ◽  
Surface Productivity ◽  
The Last Glacial

&lt;p&gt;Paleoceanographic studies reconstructing surface paleoproductivity and benthic conditions allow us to measure the effectiveness of the biological pump, an important mechanism in the global climate system. In order to assess surface productivity changes and their effect on the sea-floor environment, a multiproxy paleoceanographic analysis was conducted on the core SAT-048A (1542 m.b.s.l.), recovered from the continental slope of the southernmost Brazilian continental margin, western South Atlantic. We assessed sea surface productivity using different planktonic foraminiferal proxies: (1) the relative abundances of the species &lt;em&gt;Globigerina bulloides&lt;/em&gt; and &lt;em&gt;Globigerinita glutinata&lt;/em&gt; and (2) the &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C signal of shells of the species &lt;em&gt;Globigerinoides ruber ruber&lt;/em&gt;. To assess the organic matter (OM) flux to the seafloor, the foraminiferal planktonic:benthic ratio and the &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C signal of shells of the benthic foraminifer &lt;em&gt;Uvigerina&lt;/em&gt; spp. were used. To study dissolution effects occurring at the sea-floor, the Fragmentation Intensity (i.e., the proportion of fragments and broken foraminiferal shells), the number of planktonic foraminiferal tests per gram of dry sediment, and the CaCO&lt;sub&gt;3&lt;/sub&gt; and Sand contents of the sediment were measured. Superimposed on the climate-induced changes related to the last glacial-interglacial transition, the reconstruction indicates paleoproductivity changes synchronized with the precessional cycle. From the reconstructed data, it was possible to identify the glacial and postglacial stages: surface productivity, flux to the seafloor, and dissolution rates of planktonic foraminiferal tests where high during the glacial and low during the postglacial. Furthermore, within the glacial, enhanced productivity was associated with higher insolation values, which can be explained by increased NE summer winds that strengthened the Brazil Current transport and, in turn, promoted meandering and upwelling of the nutrient rich South Atlantic Central Water. Changes in the Atlantic Meridional Overturning Circulation and the reorganization of bottom water masses may change the CO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt; saturation levels and, consequently, influence carbonate preservation. However, the &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C values from shells of &lt;em&gt;Uvigerina&lt;/em&gt; spp. are different from present-day &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C values from dissolved inorganic carbon for the Upper Circumpolar Deep Water and the North Atlantic Deep Water, which is likely linked to varying OM fluxes. Future studies (e.g., &amp;#949;Nd in benthic Foraminifera) must quantify the effect of the reorganization of the bottom water masses on the dissolution of the planktonic foraminiferal tests, to better understand the effect of the biological pump removing carbon from the seawater and its subsequent sequestration in the seafloor sediments.&lt;/p&gt;

Modes of response of the subsurface western South Atlantic to the last glacial Dansgaard-Oeschger cycles

2020 ◽  
Author(s):  
Thiago Santos ◽  
João Ballalai ◽  
Daniel Franco ◽  
Rômulo Oliveira ◽  
Douglas Lessa ◽  
...  
Keyword(s):  
Ice Cores ◽  
South Atlantic ◽  
Meridional Overturning Circulation ◽  
Last Glacial ◽  
Mis 3 ◽  
Subsurface Layers ◽  
Basal Melting ◽  
Mis 5 ◽  
The Last Glacial ◽  
Millennial Scale

&lt;p&gt;The last glacial was an interval characterized by a sequence of abrupt millennial-scale events well documented mainly from the Greenland and Antarctica ice-cores. Although the triggers are not fully understood, most of the works agree that they occurred in consonance with oscillations in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Paleoceanographic reconstructions have shown that cold millennial-scale stadials were accompanied by high temperatures in the subsurface to intermediate waters of the Atlantic Ocean that may have acted in both the basal melting of ice-sheets and in the rapid atmospheric warming during the onset of warm interstadials. Assuming that recent transient models indicated an accentuated response of the subsurface western South Atlantic to the millennial-scale disturbances, here we present a paleoceanographic reconstruction in this area based on the deep-dwelling planktic foraminifer Globorotalia inflata. Our high-resolution oxygen isotope (d&lt;sup&gt;18&lt;/sup&gt;O) presents a sequence of millennial-scale variability that strongly resembles the structure of the Greenland Dansgaard-Oeschger cycles, mainly during Marine Isotope Stage (MIS) 5. On the other hand, during MIS 3, this millennial-scale feature is absent or weakly represented. Cross-spectral analyzes indicate a meaningful north-to-south forcing over the western South Atlantic subsurface during early-glacial. Mg/Ca-derived temperature and ice-volume free seawater d&lt;sup&gt;18&lt;/sup&gt;O (d&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;IVF-SW&lt;/sub&gt;) executed for the MIS 5 interval demonstrated that the subsurface western South Atlantic was warmer and saltier (colder and fresher) during early glacial stadial (interstadials). We hypothesized that a wide reorganization of the northward heat transport throughout the last glacial occurred, in which regions so far south as 24 &amp;#186;S worked as prominent heat reservoirs in periods of weakened AMOC during MIS 5 but not necessarily during MIS 3. Our data suggest that future impacts over the AMOC along the Brazilian margin will likely be recognized in the subsurface layers of the western South Atlantic.&lt;/p&gt;

scholarly journals Atmospheric gas records from Taylor Glacier, Antarctica, reveal ancient ice with ages spanning the entire last glacial cycle

2017 ◽  
Vol 13 (7) ◽  
pp. 943-958 ◽  
Author(s):  
Daniel Baggenstos ◽  
Thomas K. Bauska ◽  
Jeffrey P. Severinghaus ◽  
James E. Lee ◽  
Hinrich Schaefer ◽  
...  
Keyword(s):  
Large Scale ◽  
Ice Core ◽  
Ice Sheet ◽  
Last Deglaciation ◽  
Glacial Cycle ◽  
Last Glacial ◽  
Climate Reconstructions ◽  
Age Model ◽  
Taylor Glacier ◽  
The Last Glacial

Abstract. Old ice for paleo-environmental studies, traditionally accessed through deep core drilling on domes and ridges on the large ice sheets, can also be retrieved at the surface from ice sheet margins and blue ice areas. The practically unlimited amount of ice available at these sites satisfies a need in the community for studies of trace components requiring large sample volumes. For margin sites to be useful as ancient ice archives, the ice stratigraphy needs to be understood and age models need to be established. We present measurements of trapped gases in ice from Taylor Glacier, Antarctica, to date the ice and assess the completeness of the stratigraphic section. Using δ18O of O2 and methane concentrations, we unambiguously identify ice from the last glacial cycle, covering every climate interval from the early Holocene to the penultimate interglacial. A high-resolution transect reveals the last deglaciation and the Last Glacial Maximum (LGM) in detail. We observe large-scale deformation in the form of folding, but individual stratigraphic layers do not appear to have undergone irregular thinning. Rather, it appears that the entire LGM–deglaciation sequence has been transported from the interior of the ice sheet to the surface of Taylor Glacier relatively undisturbed. We present an age model that builds the foundation for gas studies on Taylor Glacier. A comparison with the Taylor Dome ice core confirms that the section we studied on Taylor Glacier is better suited for paleo-climate reconstructions of the LGM due to higher accumulation rates.

scholarly journals A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial

2019 ◽  
Vol 15 (6) ◽  
pp. 1939-1958
Author(s):  
Svante Björck ◽  
Jesper Sjolte ◽  
Karl Ljung ◽  
Florian Adolphi ◽  
Roger Flower ◽  
...  
Keyword(s):  
Southern Hemisphere ◽  
Large Scale ◽  
Ice Cores ◽  
Oceanic Islands ◽  
South Atlantic ◽  
Heat Fluxes ◽  
Meridional Overturning Circulation ◽  
Driving Mechanism ◽  
Last Glacial ◽  
The Last Glacial

Abstract. Changes in the latitudinal position and strength of the Southern Hemisphere westerlies (SHW) are thought to be tightly coupled to important climate processes, such as cross-equatorial heat fluxes, Atlantic Meridional Overturning Circulation (AMOC), the bipolar seesaw, Southern Ocean ventilation and atmospheric CO2 levels. However, many uncertainties regarding magnitude, direction, and causes and effects of past SHW shifts still exist due to lack of suitable sites and scarcity of information on SHW dynamics, especially from the last glacial. Here we present a detailed hydroclimate multiproxy record from a 36.4–18.6 kyr old lake sediment sequence on Nightingale Island (NI). It is strategically located at 37∘ S in the central South Atlantic (SA) within the SHW belt and situated just north of the marine Subtropical Front (SF). This has enabled us to assess hydroclimate changes and their link to the regional climate development as well as to large-scale climate events in polar ice cores. The NI record exhibits a continuous impact of the SHW, recording shifts in both position and strength, and between 36 and 31 ka the westerlies show high latitudinal and strength-wise variability possibly linked to the bipolar seesaw. This was followed by 4 kyr of slightly falling temperatures, decreasing humidity and fairly southerly westerlies. After 27 ka temperatures decreased 3–4 ∘C, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly-positioned SHW seem to be related to periods of increased CO2 outgassing from the ocean, while changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, our results show that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial. Finally we can conclude that multiproxy lake records from oceanic islands have the potential to record atmospheric variability coupled to large-scale climate shifts over vast oceanic areas.

scholarly journals Meridional overturning circulation in the South Atlantic at the last glacial maximum

2006 ◽  
Vol 7 (10) ◽  
pp. n/a-n/a ◽  
Author(s):  
Jean Lynch-Stieglitz ◽  
William B. Curry ◽  
Delia W. Oppo ◽  
Ulysses S. Ninneman ◽  
Christopher D. Charles ◽  
...  
Keyword(s):  
Last Glacial Maximum ◽  
South Atlantic ◽  
Glacial Maximum ◽  
Meridional Overturning Circulation ◽  
The South ◽  
Overturning Circulation ◽  
Last Glacial ◽  
Meridional Overturning ◽  
The Last Glacial Maximum ◽  
The Last Glacial
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