scholarly journals Millennial changes in North Atlantic oxygen concentrations

2015 ◽  
Vol 12 (15) ◽  
pp. 12947-12973 ◽  
Author(s):  
B. A. A. Hoogakker ◽  
D. J. R. Thornalley ◽  
S. Barker
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Bottom Water ◽  
Water Mass ◽  
Intermediate Water ◽  
Northeast Atlantic ◽  
The North ◽  
Cold Events ◽  
The North Atlantic ◽  
Oxygen Concentrations

Abstract. Glacial–interglacial changes in bottom water oxygen concentrations [O2] in the deep Northeast Atlantic have been linked to decreased ventilation relating to changes in ocean circulation and the biological pump (Hoogakker et al., 2015). In this paper we discuss seawater [O2] changes in relation to millennial climate oscillations in the North Atlantic ocean over the last glacial cycle, using bottom water [O2] reconstructions from 2 cores: (1) MD95-2042 from the deep northeast Atlantic (Hoogakker et al., 2015), and (2) ODP 1055 from the intermediate northwest Atlantic. Deep northeast Atlantic core MD95-2042 shows decreased bottom water [O2] during millennial scale cool events, with lowest bottom water [O2] of 170, 144, and 166 ± 17 μmol kg−1 during Heinrich ice rafting events H6, H4 and H1. Importantly, at intermediate core ODP 1055 bottom water [O2] was lower during parts of Marine Isotope Stage 4 and millennial cool events, with lowest values of 179 and 194 μmol kg−1 recorded during millennial cool events C21 and a cool event following Dansgaard–Oeschger event 19. Our reconstructions agree with previous model simulations suggesting that glacial cold events may be associated with lower seawater [O2] across the North Atlantic below ~1 km (Schmittner et al., 2007), although in our reconstructions the changes are less dramatic. The decreases in bottom water [O2] during North Atlantic Heinrich events and earlier cold events at the deep site can be linked to water mass changes in relation to ocean circulation changes, and possibly productivity changes. At the intermediate depth site a strong North Atlantic Intermediate Water cell precludes water mass changes as a cause for decreased bottom water [O2]. Instead we propose that the lower bottom [O2] there can be linked to productivity changes through increased export of organic material from the surface ocean.

scholarly journals Millennial changes in North Atlantic oxygen concentrations

2016 ◽  
Vol 13 (1) ◽  
pp. 211-221 ◽  
Author(s):  
B. A. A. Hoogakker ◽  
D. J. R. Thornalley ◽  
S. Barker
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Bottom Water ◽  
Water Mass ◽  
Intermediate Depth ◽  
Northeast Atlantic ◽  
The North ◽  
Cold Events ◽  
The North Atlantic ◽  
Oxygen Concentrations

Abstract. Glacial–interglacial changes in bottom water oxygen concentrations [O2] in the deep northeast Atlantic have been linked to decreased ventilation relating to changes in ocean circulation and the biological pump (Hoogakker et al., 2015). In this paper we discuss seawater [O2] changes in relation to millennial climate oscillations in the North Atlantic over the last glacial cycle, using bottom water [O2] reconstructions from 2 cores: (1) MD95-2042 from the deep northeast Atlantic (Hoogakker et al., 2015) and (2) ODP (Ocean Drilling Program) Site 1055 from the intermediate northwest Atlantic. The deep northeast Atlantic core MD95-2042 shows decreased bottom water [O2] during millennial-scale cool events, with lowest bottom water [O2] of 170, 144, and 166 ± 17 µmol kg−1 during Heinrich ice rafting events H6, H4, and H1. Importantly, at intermediate depth core ODP Site 1055, bottom water [O2] was lower during parts of Marine Isotope Stage 4 and millennial cool events, with the lowest values of 179 and 194 µmol kg−1 recorded during millennial cool event C21 and a cool event following Dansgaard–Oeschger event 19. Our reconstructions agree with previous model simulations suggesting that glacial cold events may be associated with lower seawater [O2] across the North Atlantic below  ∼ 1 km (Schmittner et al., 2007), although in our reconstructions the changes are less dramatic. The decreases in bottom water [O2] during North Atlantic Heinrich events and earlier cold events at the two sites can be linked to water mass changes in relation to ocean circulation changes and possibly productivity changes. At the intermediate depth site a possible strong North Atlantic Intermediate Water cell would preclude water mass changes as a cause for decreased bottom water [O2]. Instead, we propose that the lower bottom [O2] there can be linked to productivity changes through increased export of organic material from the surface ocean and its subsequent remineralization in the water column and the sediment.

scholarly journals Water mass distributions and transports for the 2014 GEOVIDE cruise in the North Atlantic

2018 ◽  
Vol 15 (7) ◽  
pp. 2075-2090 ◽  
Author(s):  
Maribel I. García-Ibáñez ◽  
Fiz F. Pérez ◽  
Pascale Lherminier ◽  
Patricia Zunino ◽  
Herlé Mercier ◽  
...  
Keyword(s):  
North Atlantic ◽  
Water Mass ◽  
Water Levels ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Intermediate Water ◽  
North East ◽  
The North ◽  
The North Atlantic

Abstract. We present the distribution of water masses along the GEOTRACES-GA01 section during the GEOVIDE cruise, which crossed the subpolar North Atlantic Ocean and the Labrador Sea in the summer of 2014. The water mass structure resulting from an extended optimum multiparameter (eOMP) analysis provides the framework for interpreting the observed distributions of trace elements and their isotopes. Central Waters and Subpolar Mode Waters (SPMW) dominated the upper part of the GEOTRACES-GA01 section. At intermediate depths, the dominant water mass was Labrador Sea Water, while the deep parts of the section were filled by Iceland–Scotland Overflow Water (ISOW) and North-East Atlantic Deep Water. We also evaluate the water mass volume transports across the 2014 OVIDE line (Portugal to Greenland section) by combining the water mass fractions resulting from the eOMP analysis with the absolute geostrophic velocity field estimated through a box inverse model. This allowed us to assess the relative contribution of each water mass to the transport across the section. Finally, we discuss the changes in the distribution and transport of water masses between the 2014 OVIDE line and the 2002–2010 mean state. At the upper and intermediate water levels, colder end-members of the water masses replaced the warmer ones in 2014 with respect to 2002–2010, in agreement with the long-term cooling of the North Atlantic Subpolar Gyre that started in the mid-2000s. Below 2000 dbar, ISOW increased its contribution in 2014 with respect to 2002–2010, with the increase being consistent with other estimates of ISOW transports along 58–59° N. We also observed an increase in SPMW in the East Greenland Irminger Current in 2014 with respect to 2002–2010, which supports the recent deep convection events in the Irminger Sea. From the assessment of the relative water mass contribution to the Atlantic Meridional Overturning Circulation (AMOC) across the OVIDE line, we conclude that the larger AMOC intensity in 2014 compared to the 2002–2010 mean was related to both the increase in the northward transport of Central Waters in the AMOC upper limb and to the increase in the southward flow of Irminger Basin SPMW and ISOW in the AMOC lower limb.

scholarly journals Abrupt cold events in the North Atlantic in a transient Holocene simulation

2017 ◽  
Author(s):  
Andrea Klus ◽  
Matthias Prange ◽  
Vidya Varma ◽  
Louis Bruno Tremblay ◽  
Michael Schulz
Keyword(s):  
Sea Ice ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Surface Cooling ◽  
Climate System Model ◽  
The North ◽  
Cold Events ◽  
The North Atlantic ◽  
Polar Ocean

Abstract. Abrupt cold events have been detected in numerous North Atlantic climate records from the Holocene. Several mechanisms have been discussed as possible triggers for these climate shifts persisting decades to centuries. Here, we describe two cold events that occurred during an orbitally forced transient Holocene simulation using the Community Climate System Model version 3. Both events occurred during the late Holocene (event 1 referring to 4305–4267 BP and event 2 referring to 3046–3018 BP) and were characterized by substantial surface cooling (−2.7 and −2.2 °C, respectively) and freshening (−0.7 and −0.6 PSU, respectively) as well as severe sea ice advance east of Newfoundland and south of Greenland. Sea ice even reached the Iceland Basin in the northeastern Atlantic at the climaxes of the cold events. Convection and deep-water formation in the northwestern Atlantic collapsed during the events, while the Atlantic meridional overturning circulation was not significantly affected. The events were triggered by prolonged phases of a positive North Atlantic Oscillation which, through changes in surface winds, caused substantial changes in the sub-polar ocean circulation and associated freshwater transports, resulting in a weakening of the sub-polar gyre. Our results suggest a possible mechanism by which abrupt cold events in the North Atlantic region may be triggered by internal climate variability without the need of an external (e.g. solar or volcanic) forcing.

scholarly journals Abrupt cold events in the North Atlantic Ocean in a transient Holocene simulation

2018 ◽  
Vol 14 (8) ◽  
pp. 1165-1178 ◽  
Author(s):  
Andrea Klus ◽  
Matthias Prange ◽  
Vidya Varma ◽  
Louis Bruno Tremblay ◽  
Michael Schulz
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Meridional Overturning Circulation ◽  
Surface Cooling ◽  
Climate System Model ◽  
Internal Climate Variability ◽  
The North ◽  
Cold Events ◽  
The North Atlantic

Abstract. Abrupt cold events have been detected in numerous North Atlantic climate records from the Holocene. Several mechanisms have been discussed as possible triggers for these climate shifts persisting decades to centuries. Here, we describe two abrupt cold events that occurred during an orbitally forced transient Holocene simulation using the Community Climate System Model version 3. Both events occurred during the late Holocene (4305–4267 BP and 3046–3018 BP for event 1 and event 2, respectively). They were characterized by substantial surface cooling (−2.3 and −1.8 ∘C, respectively) and freshening (−0.6 and −0.5 PSU, respectively) as well as severe sea ice advance east of Newfoundland and south of Greenland, reaching as far as the Iceland Basin in the northeastern Atlantic at the climaxes of the cold events. Convection and deep-water formation in the northwestern Atlantic collapsed during the events, while the Atlantic Meridional Overturning Circulation was not substantially affected (weakening by only about 10 % and 5 %, respectively). The events were triggered by prolonged phases of a positive North Atlantic Oscillation that caused substantial changes in the subpolar ocean circulation and associated freshwater transports, resulting in a weakening of the subpolar gyre. Our results suggest a possible mechanism by which abrupt cold events in the North Atlantic region may be triggered by internal climate variability without the need of an external (e.g., solar or volcanic) forcing.

scholarly journals Evolution of <sup>231</sup>Pa and <sup>230</sup>Th in overflow waters of the North Atlantic

2018 ◽  
Vol 15 (23) ◽  
pp. 7299-7313 ◽  
Author(s):  
Feifei Deng ◽  
Gideon M. Henderson ◽  
Maxi Castrillejo ◽  
Fiz F. Perez ◽  
Reiner Steinfeldt
Keyword(s):  
Water Column ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Water Mass ◽  
Deep Ocean ◽  
Water Age ◽  
Meridional Transport ◽  
The North ◽  
Basin Scale ◽  
The North Atlantic

Abstract. Many palaeoceanographic studies have sought to use the 231Pa∕230Th ratio as a proxy for deep ocean circulation rates in the North Atlantic. As of yet, however, no study has fully assessed the concentration of, or controls on, 230Th and 231Pa in waters immediately following ventilation at the start of Atlantic meridional overturning. To that end, full water-column 231Pa and 230Th concentrations were measured along the GEOVIDE section, sampling a range of young North Atlantic deep waters. 230Th and 231Pa concentrations in the water column are lower than those observed further south in the Atlantic, ranging between 0.06 and 12.01 µBq kg−1 and between 0.37 and 4.80 µBq kg−1, respectively. Both 230Th and 231Pa profiles generally increase with water depth from surface to deep water, followed by decrease near the seafloor, with this feature most pronounced in the Labrador Sea (LA Sea) and Irminger Sea (IR Sea). Assessing this dataset using extended optimum multi-parameter (eOMP) analysis and CFC-based water mass age indicates that the low values of 230Th and 231Pa in water near the seafloor of the LA Sea and IR Sea are related to the young waters present in those regions. The importance of water age is confirmed for 230Th by a strong correlation between 230Th and water mass age (though this relationship with age is less clear for 231Pa and the 231Pa∕230Th ratio). Scavenged 231Pa and 230Th were estimated and compared to their potential concentrations in the water column due to ingrowth. This calculation indicates that more 230Th is scavenged (∼80 %) than 231Pa (∼40 %), consistent with the relatively higher particle reactivity of 230Th. Enhanced scavenging for both nuclides is demonstrated near the seafloor in young overflow waters. Calculation of the meridional transport of 230Th and 231Pa with this new GEOVIDE dataset enables a complete budget for 230Th and 231Pa for the North Atlantic. Results suggest that net transport southward of 230Th and 231Pa across GEOVIDE is smaller than transport further south in the Atlantic, and indicate that the flux to sediment in the North Atlantic is equivalent to 96 % of the production of 230Th and 74 % of the production for 231Pa. This result confirms a significantly higher advective loss of 231Pa to the south relative to 230Th and supports the use of 231Pa∕230Th to assess meridional transport at a basin scale.

scholarly journals Evolution of <sup>231</sup>Pa and <sup>230</sup>Th in overflow waters of the North Atlantic

2018 ◽  
Author(s):  
Feifei Deng ◽  
Gideon M. Henderson ◽  
Maxi Castrillejo ◽  
Fiz F. Perez
Keyword(s):  
Water Column ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Water Mass ◽  
Deep Ocean ◽  
Water Age ◽  
Meridional Transport ◽  
The North ◽  
Basin Scale ◽  
The North Atlantic

Abstract. Many paleoceanographic studies have sought to use the 231Pa / 230Th ratio as a proxy for deep ocean circulation rates in the North Atlantic. As yet, however, no study has fully assessed the concentration of, or controls on, 230Th and 231Pa in waters immediately following ventilation at the start of Atlantic meridional overturning. To that end, full water-column 231Pa and 230Th concentrations were measured along the GEOVIDE section, sampling a range of young North Atlantic deep waters. Th-230 and 231Pa concentrations in the water column are lower than those observed further south in the Atlantic, ranging between 0.004 and 0.738 dpm/1000l, and between 0.023 and 0.295 dpm/1000l, respectively. Both 230Th and 231Pa profiles generally increase with water depth from surface to deep water, followed by decrease near the seafloor, with this feature most pronounced in the Labrador Sea (LA Sea) and Irminger Sea (IR Sea). Analyzing this dataset with Extended Optimum Multi-Parameter (eOMP) Analysis and CFC-based water mass age indicates that the low values of 230Th and 231Pa in water near the seafloor of the LA Sea and IR Sea are related to the young waters present in those regions. This importance of water age is confirmed for 230Th by a strong correlation between 230Th and water mass age (though this relationship is less clear, for 231Pa and 231Pa / 230Th ratio). Scavenged 231Pa and 230Th were estimated and compared to their Potential Total concentrations in the water column. The result shows that more 230Th is scavenged (~ 80 %) relative to 231Pa (~ 40 %), consistent with the relatively higher particle-reactivity of 230Th. Enhanced scavenging for both nuclides is demonstrated near the seafloor in young overflow waters. Calculation of meridional transport of 230Th and 231Pa with this new GEOVIDE dataset enables a complete budget for 230Th and 231Pa for the North Atlantic. Results suggest that net transport southward of 230Th and 231Pa across GEOVIDE is smaller than transport further south in the Atlantic, and indicates that the flux to sediment in the North Atlantic is equivalent to 96 % of the production of 230Th, and 77 % of the production for 231Pa. This result confirms a significantly higher advective loss of 231Pa to the south relative to 230Th and supports the use of 231Pa / 230Th to assess meridional transport at a basin scale.

scholarly journals Surface and deep water circulation in late Cretaceous North Atlantic greenhouse ocean

2009 ◽  
Author(s):  
◽  
Carolina Isaza Londoño
Keyword(s):  
Ocean Circulation ◽  
Late Cretaceous ◽  
North Atlantic ◽  
Deep Water ◽  
Water Mass ◽  
Deep Ocean ◽  
Water Circulation ◽  
Regional Warming ◽  
The North ◽  
The North Atlantic

This research provides the first of its kind empirical data regarding the evolution of Maastrichtian surface to deep ocean circulation in the North Atlantic. Differences in foraminiferal abundances and oxygen and carbon isotopic ratios of bulk carbonate and foraminifera between two Ocean Drilling Program Sites in the subtropical North Atlantic indicate a sharp water mass boundary was a relatively stable and persistent feature of the Maastrichtian North Atlantic despite significant regional warming across the interval. Neodymium isotopes of fish debris, on the other hand, indicate significant changes in intermediate and deep water circulation through the Late Cretaceous and especially during the Maastrichtian. During the Cenomanian-Campanian interval at least three different deep water masses were active in the North Atlantic including one formed by downwelling of warm saline waters in the Demerara Rise region. During the Campanian-Maastrichtian, low-latitude-sourced waters seem to have reached abyssal depths, but from the mid-Maastrichtian on, this water mass seems to have declined in importance. From the mid-Danian on, we found evidence for only one water mass (plausibly sourced in the northern North Atlantic, as it is today) at bathyal and abyssal depths in the North Atlantic. Our data demonstrate that surface and, especially, intermediate and deep water circulation patterns are an important (and measurable) variable that helps determine greenhouse temperature distributions on regional and global scales.

The Role of Oscillating Southern Hemisphere Westerly Winds: Global Ocean Circulation

2020 ◽  
Vol 33 (6) ◽  
pp. 2111-2130
Author(s):  
Woo Geun Cheon ◽  
Jong-Seong Kug
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
North Pacific ◽  
Global Ocean ◽  
The North ◽  
Westerly Winds ◽  
The North Atlantic ◽  
Global Ocean Circulation ◽  
The Antarctic ◽  
The North Pacific

AbstractIn the framework of a sea ice–ocean general circulation model coupled to an energy balance atmospheric model, an intensity oscillation of Southern Hemisphere (SH) westerly winds affects the global ocean circulation via not only the buoyancy-driven teleconnection (BDT) mode but also the Ekman-driven teleconnection (EDT) mode. The BDT mode is activated by the SH air–sea ice–ocean interactions such as polynyas and oceanic convection. The ensuing variation in the Antarctic meridional overturning circulation (MOC) that is indicative of the Antarctic Bottom Water (AABW) formation exerts a significant influence on the abyssal circulation of the globe, particularly the Pacific. This controls the bipolar seesaw balance between deep and bottom waters at the equator. The EDT mode controlled by northward Ekman transport under the oscillating SH westerly winds generates a signal that propagates northward along the upper ocean and passes through the equator. The variation in the western boundary current (WBC) is much stronger in the North Atlantic than in the North Pacific, which appears to be associated with the relatively strong and persistent Mindanao Current (i.e., the southward flowing WBC of the North Pacific tropical gyre). The North Atlantic Deep Water (NADW) formation is controlled by salt advected northward by the North Atlantic WBC.

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