Data/hora: 09/06/2024 12:30:48

Provedor de dados: | 5
País: | France

Título:

Impact of deglacial and Holocene AMOC changes on the mixed layer and deepwater hydrography of the subtropical North Atlantic

Auswirkungen deglazialer und holozäner AMOC Veränderungen auf die Oberflächen- und Tiefenwasserhydrographie im subtropischen Nordatlantik

Autores: | Respschläger, Janne.
Data: | 2013-08-28
Ano: | 2013
Palavras-chave: | Paleoceanography; AMOC; Subtropical gyre; Deglacial; Multiproxy SST; BWT; Stable isotopes; Planktonic and benthic foraminifera.
Resumo: | The Atlantic meridional overturning circulation (AMOC) transports warm saline water from low to high northern latitudes where it cools, sinks down and returns southward as deepwater flow. By the heat transfer from low to high latitudes, AMOC contributes in balancing the global energy budget and thus is an important part of global climate system. Within this study the impact of AMOC changes on surface and deepwater hydrography of the subtropical North Atlantic is investigated by the use of high-resolution sediment cores from a coring site located south of the Azores at the eastern flank of the Mid Atlantic Ridge. The Azores coring site is situated in the subtropical eastern North Atlantic (NA) at the boundary between warm Subtropical Gyre Water (STG) and temperate East North Atlantic Water (ENAW) at the surface. At depth the site is located at the interface between southern sourced Lower Deep Water (LDW) and North East Atlantic Deep Water (NEADW) that consists of a mixture of northern sourced Iceland-Scotland Overflow Water (ISOW) and Labrador Sea Water (LSW). Due to its location at water mass boundaries the coring position is ideal to reconstruct coupled surface and deepwater changes. Deglacial records in centennial to decadal resolution of changes in the mixed layer and the deepwater are established. Multiproxy sea surface temperature (SST) reconstructions using Mg/Ca surface of planktonic surface and subsurface dwelling foraminifera, the alkenone undersaturation index UK´37 and foraminifera transfer function (SIMMAX) are used to gain a reliable deglacial SST records from the subtropcial NA and to decipher environmental imprints other than temperature on SST reconstructions. The combination of these SST records with stable oxygen isotope records (δ18O) from planktonic surface and subsurface dwelling foraminifera are used to derive changes in the mixed layer salinity (18Ow-ice). Additionally, variations in the STG position are reconstructed by the use of foraminifera abundance data. Deglacial changes in deepwater composition are traced by its ventilation, temperature and salinity by the coupled use of stable isotope (δ18O and δ13C) and Mg/Ca bottom water temperature (BWT) reconstructions from benthic foraminifera. This new data indicates that the position of the NA subtropical Gyre (STG) was coupled with AMOC strength over the last deglaciation and the early to mid Holocene (16 -7 ka BP). During cold deglacial and Holocene weak AMOC phases (Heinrich event 1 (H1), Younger Dryas (YD), Preboreal and 8.2 event) the STG was displaced southward. This STG displacement most probably was driven by meltwater that reached southward to the Azores coring site (38°N) as indicated by salinity reconstructions (18Ow-ice) that indicate at least a salinity decrease of 1‰ Standard Mean Ocean Water (SMOW) for the H1 and 8.2 events. Additionally a southward extension of temperate ENAW led to a SST cooling of 4.5-6°C in comparison to modern values. On the contrary the presence of relatively warm subsurface water (14°C) during the YD might indicate subsurface heat storage during this event. The new high-resolution BWT, ventilation and δ18Ow-ice record from the subtropical eastern NA indicates that the deepwater composition in the eastern North Atlantic basin changed over the last 15 ka BP in parallel to deglacial changes in surface water hydrography. A general evolution toward a warmer better ventilated deepwater was disrupted during the H1 event, the YD and the early Holocene by a less ventilated (δ13C values between 0-0.5‰), cool (1.5°C minimum BWT) and freshened water (0.5 ‰ δ18Ow-ice depletion). The parallel evolution of BWT, δ13C and δ18Ow-ice with changes in deep- and surface water compositions in the subpolar NA and with the Greenland ice core record NGRIP indicates that variations in deepwater composition over the last 15 ka BP were triggered by high northern latitude climatic processes. This new results contradict the idea that southern sourced deepwater entered the subpolar NA during cold events. In consequence the new results would imply that the NA changes contributed even to deglacial changes in the Antarctic bottom water composition and finally had an impact on changes in the deepwater stratification mode as is discussed in a new conceptual model.
Tipo: | Text
Idioma: | Inglês

Identificador:

https://archimer.ifremer.fr/doc/00493/60444/63889.pdf

https://archimer.ifremer.fr/doc/00493/60444/

Editor: | University of Kiel, Mathematics and Natural Sciences
Formato: | application/pdf

Fonte:

info:eu-repo/semantics/openAccess

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