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- Cramwinckel, Margot J., et al.
(författare)
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A Warm, Stratified, and Restricted Labrador Sea Across the Middle Eocene and Its Climatic Optimum
- 2020
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Ingår i: Paleoceanography and Paleoclimatology. - 2572-4517 .- 2572-4525. ; 35:10
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Tidskriftsartikel (refereegranskat)abstract
- Several studies indicate that North Atlantic Deep Water (NADW) formation might have initiated during the globally warm Eocene (56–34 Ma). However, constraints on Eocene surface ocean conditions in source regions presently conducive to deep water formation are sparse. Here we test whether ocean conditions of the middle Eocene Labrador Sea might have allowed for deep water formation by applying (organic) geochemical and palynological techniques, on sediments from Ocean Drilling Program (ODP) Site 647. We reconstruct a long‐term sea surface temperature (SST) drop from ~30°C to ~27°C between 41.5 to 38.5 Ma, based on TEX86. Superimposed on this trend, we record ~2°C warming in SST associated with the Middle Eocene Climatic Optimum (MECO; ~40 Ma), which is the northernmost MECO record as yet, and another, likely regional, warming phase at ~41.1 Ma, associated with low‐latitude planktic foraminifera and dinoflagellate cyst incursions. Dinoflagellate cyst assemblages together with planktonic foraminiferal stable oxygen isotope ratios overall indicate low surface water salinities and strong stratification. Benthic foraminifer stable carbon and oxygen isotope ratios differ from global deep ocean values by 1–2‰ and 2–4‰, respectively, indicating geographic basin isolation. Our multiproxy reconstructions depict a consistent picture of relatively warm and fresh but also highly variable surface ocean conditions in the middle Eocene Labrador Sea. These conditions were unlikely conducive to deep water formation. This implies either NADW did not yet form during the middle Eocene or it formed in a different source region and subsequently bypassed the southern Labrador Sea.
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| 2. |
- Houben, Alexander J. P., et al.
(författare)
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The Eocene-Oligocene transition : Changes in sea level, temperature or both?
- 2012
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier BV. - 0031-0182 .- 1872-616X. ; 335, s. 75-83
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Tidskriftsartikel (refereegranskat)abstract
- The Eocene-Oligocene Transition (EOT similar to 34 Ma) reflects the onset of major Antarctic glaciation. The primary geochemical signature of the EOT is two similar to 300 kyr spaced shifts in increasing deep-sea oxygen isotope values, possibly reflecting both global cooling a nd/or increasing ice volume. A way to assess the respective contribution of continental ice is to quantify concomitant glacio-eustatic sea level change. This is usually expressed in relatively shallow marine depositional settings. One potentially suitable region is in the Vicentinian Alps, NE Italy, where marginal marine deposits document sea level changes during the;EOT. By correlating stable isotope-, bio- and magnetostratigraphic information between three distant regions, we are able to relate the shallow marine sections to the Pacific oxygen isotope record from Ocean Drilling Program (ODP) Site 1218 of Coxall et al. (2005). Microfacies, sedimentological, and biotic analysis suggests that associated with the first isotope shift (EOT-1) sea level fell similar to 20 m, and with the ultimate shift, the Oligocene Isotope Event 1 (Oi-1) sea level fell some 50-60 m. Distribution patterns of temperature sensitive dinoflagellates from a coeval central Italian section reveal that the early stages of the EOT were accompanied by sea surface cooling, whereas no sustained cooling is noted in association with the Oi-1. This suggests that the initial EOT shift(s) reflect a mixed signal of ice volume and temperature whereas the Oi-1 primarily reflects expansion of the Antarctic cryosphere.
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