| 1. |
- Cramwinckel, Margot J., et al.
(författare)
-
A Warm, Stratified, and Restricted Labrador Sea Across the Middle Eocene and Its Climatic Optimum
- 2020
-
Ingår i: Paleoceanography and Paleoclimatology. - 2572-4517 .- 2572-4525. ; 35:10
-
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.
|
|
| 2. |
- Henderiks, Jorijntje, et al.
(författare)
-
Shifts in Phytoplankton Composition and Stepwise Climate Change During the Middle Miocene
- 2020
-
Ingår i: Paleoceanography and Paleoclimatology. - : American Geophysical Union (AGU). - 2572-4517 .- 2572-4525. ; 35:8
-
Tidskriftsartikel (refereegranskat)abstract
- The abundance and composition of modern phytoplankton are primarily related to equator‐to‐pole temperature gradients and global ocean circulation, which in turn determine the availability of nutrients in the photic zone. The nutricline is found at greater depths in warm, tropical waters, whereas more vigorous surface mixing in higher latitudes (seasonally) enhances nutrient availability and primary productivity. Ocean temperatures were ~7°C higher during the middle Miocene Climatic Optimum (MCO; ~16.9–14.7 million years ago, Ma), which was followed by Antarctic glaciation and global cooling during the middle Miocene Climate transition (MMCT; 14.7–13.8 Ma). Four decades ago, Haq (1980, https://doi.org.10.2307/1485353) already related migration patterns of low‐latitude versus high‐latitude calcareous nannoplankton in the Atlantic Ocean to major climatic fluctuations during the Miocene. Here, we detail and discuss the macroevolutionary patterns and processes across the middle Miocene (~16.5–11 Ma) at five deep sea sites on a north‐south transect in the Atlantic Ocean (57°N to 28°S). We show that the major cooling step toward the modern “icehouse” world impacted coccolithophore communities at all latitudes. Contrary to previous observations suggesting that tropical sites showed little change and that midlatitudes were the most sensitive recorders of climate change across the MMCT, we show that all sites recorded a marked diversification and increase in abundance of reticulofenestrids. Global cooling and related increased meridional overturning circulation are implicated as likely forcings for this macroevolutionary step toward establishing modern coccolithophore communities that are dominated by eurythermal and eurytrophic species such as Emiliania huxleyi.Plain Language SummaryHow will marine plankton communities respond to a much warmer world than today? How fast or slow would such changes be? We can learn valuable lessons from the fossil record of coccolithophores that represent a prominent phytoplankton group in both past and modern oceans. Changes in the composition of fossil assemblages show that species tracked past climate change on submillion year scales in the Atlantic Ocean. When ocean temperatures were ~7°C warmer during the middle Miocene (~16 million years ago), tropical species reached into the midlatitudes (42°N) of the North Atlantic. When the Antarctic ice sheet grew larger and the oceans cooled after ~14.7 million years ago, a group known as the reticulofenestrids diversified and became the most common everywhere, also in the tropics. Apparently, this group maintained high fitness in an “icehouse” world with greater latitudinal temperature contrasts, more vigorous ocean circulation, and higher nutrient availability in the photic zone. Indeed, their cosmopolitan distribution implies broad temperature and other environmental tolerances, similar to the group's latest descendant, the modern species Emiliania huxleyi that also displays a wide genetic and ecophenotypic diversity.
|
|
| 3. |
- Karatsolis, Boris-Theofanis, et al.
(författare)
-
The late Miocene to early Pliocene “Humid Interval” on the NW Australian shelf : disentangling climate forcing from regional basin evolution
- 2020
-
Ingår i: Paleoceanography and Paleoclimatology. - : American Geophysical Union. - 2572-4517 .- 2572-4525. ; 35:9
-
Tidskriftsartikel (refereegranskat)abstract
- Pre-Quaternary paleoclimate studies in Australia mainly focus on terrestrial records from the southeastern part of the continent. IODP Expedition 356 drilled on the northwestern Australian shelf, yielding Miocene-Pleistocene paleoclimate records in an area where climate archives are scarce. Postexpedition research revealed a dry-to-humid transition across the latest Miocene and early Pliocene (start of the "Humid Interval"). However, the complex tectonic history of the area makes these interpretations challenging. In this study, we investigate late Miocene to early Pliocene sediment cores from two sites that are only 100 km apart but situated in two adjacent basins (Northern Carnarvon and Roebuck Basins). Combining lithofacies study, time series analysis of potassium content (K wt%), and calcareous nannofossil abundance counts (N/g), this work disentangles the complex interplay between basin evolution and climate change between 6.1 and 4 Ma. Overall, the investigated proxies show high correlation between both sites, except during 6.1-5.7 Ma. During this interval, Site U1463 records a gradual increase in K wt%, correlated with basin deepening, whereas Site U1464 records an abrupt rise in K wt% at similar to 6 Ma. We explain this diachronicity by differential basin subsidence. The tectonic interplay with our paleorecords makes it difficult to pinpoint the exact onset of the "Humid Interval," but we conclude that K wt% and coccolith abundances at Site U1464 indicate that a fluvial deposition system was already established since at least 6 Ma. This age is consistent with data supporting a southward movement of the Intertropical Convergence Zone rain belt at similar to 7 Ma.
|
|
| 4. |
- Kim, Sora, et al.
(författare)
-
Probing the ecology and climate of the Eocene Southern Ocean with sand tiger sharks Striatolamia macrota
- 2020
-
Ingår i: Paleoceanography and Paleoclimatology. - : American Geophysical Union (AGU). - 2572-4517 .- 2572-4525. ; 35:12
-
Tidskriftsartikel (refereegranskat)abstract
- Many explanations for Eocene climate change focus on the Southern Ocean—where tectonics influenced oceanic gateways, ocean circulation reduced heat transport, and greenhouse gas declines prompted glaciation. To date, few studies focus on marine vertebrates at high latitudes to discern paleoecological and paleoenvironmental impacts of this climate transition. The Tertiary Eocene La Meseta (TELM) Formation has a rich fossil assemblage to characterize these impacts; Striatolamia macrota, an extinct (†) sand tiger shark, is abundant throughout the La Meseta Formation. Body size is often tracked to characterize and integrate across multiple ecological dimensions. †S. macrota body size distributions indicate limited changes during TELMs 2–5 based on anterior tooth crown height (n = 450, mean = 19.6 ± 6.4 mm). Similarly, environmental conditions remained stable through this period based on δ18OPO4 values from tooth enameloid (n = 42; 21.5 ± 1.6‰), which corresponds to a mean temperature of 22.0 ± 4.0°C. Our preliminary εNd (n = 4) results indicate an early Drake Passage opening with Pacific inputs during TELM 2–3 (45–43 Ma) based on single unit variation with an overall radiogenic trend. Two possible hypotheses to explain these observations are (1) †S. macrota modified its migration behavior to ameliorate environmental changes related to the Drake Passage opening, or (2) the local climate change was small and gateway opening had little impact. While we cannot rule out an ecological explanation, a comparison with climate model results suggests that increased CO2 produces warm conditions that also parsimoniously explain the observations.
|
|
| 5. |
- Reghellin, Daniele, et al.
(författare)
-
Understanding Bulk Sediment Stable Isotope Records in the Eastern Equatorial Pacific, From Seven Million Years Ago to Present Day
- 2020
-
Ingår i: Paleoceanography and Paleoclimatology. - 2572-4517 .- 2572-4525. ; 35:2
-
Tidskriftsartikel (refereegranskat)abstract
- Stable isotope (delta C-13 and delta O-18) records of bulk marine sediment carry information on past carbon cycling and oceanography, but origins and interpretations remain uncertain because such signals represent mixtures of different biogenic components, each with potential offsets from primary parameters. Studies of Neogene sediment from the eastern equatorial Pacific (EEP) exemplify this issue, because stable isotope records of bulk sediment and foraminifera at different sites exhibit similarities and differences in absolute value that somehow relate to depositional age. Here we measure delta C-13 and delta O-18 of bulk carbonate, two fine-grained fractions (<63 and <20 mu m), mixed-species planktic and benthic foraminifera, and foraminifera fragments from sediments deposited over four time intervals within the last 7 Ma at ODP Site 851. These data are compared to published delta C-13 and delta O-18 records of multiple single-species planktic foraminifera from the same site and benthic foraminifera from an adjacent site. Bulk sediment delta C-13 and delta O-18 records represent a mixed signal dominated by reticulofenestrid coccolith calcite but modified by variable amounts of different foraminifera. Similarities and differences between stable isotope records result from temporal changes in water chemistry and temperature, depths of calcite precipitation, and vital effects that impact fractionation of various biogenic components. The remarkable correlation of bulk stable isotope records within the EEP suggests that several factors change collectively over time across a broad oceanographic region. Ideally, multiple stable isotope records coupled with other proxy measurements might lead to an internally consistent paleoceanographic perspective of the EEP since the late Miocene.
|
|
| 6. |
- Smith, Rebecca A., et al.
(författare)
-
Plio-Pleistocene Indonesian Throughflow Variability Drove Eastern Indian Ocean Sea Surface Temperatures
- 2020
-
Ingår i: PALEOCEANOGRAPHY AND PALEOCLIMATOLOGY. - : American Geophysical Union (AGU). - 2572-4517 .- 2572-4525. ; 35:10
-
Tidskriftsartikel (refereegranskat)abstract
- Ocean gateways facilitate circulation between ocean basins, thereby impacting global climate. The Indonesian Gateway transports water from the Pacific to the Indian Ocean via the Indonesian Throughflow (ITF) and drives the strength and intensity of the modern Leeuwin Current, which carries warm equatorial waters along the western coast of Australia to higher latitudes. Therefore, ITF dynamics are a vital component of global thermohaline circulation. Plio-Pleistocene changes in ITF behavior and Leeuwin Current intensity remain poorly constrained due to a lack of sedimentary records from regions under its influence. Here, organic geochemical proxies are used to reconstruct sea surface temperatures on the northwest Australian shelf at IODP Site U1463, downstream of the ITF outlet and under the influence of the Leeuwin Current. Our records, based on TEX86 and the long-chain diol index, provide insight into past ITF variability (3.5-1.5 Ma) and confirm that sea surface temperature exerted a control on Australian continental hydroclimate. A significant TEX86 cooling of similar to 5 degrees C occurs within the mid-Pliocene Warm Period (3.3-3.1 Ma) suggesting that this interval was characterized by SST fluctuations at Site U1463. A major feature of both the TEX86 and long-chain diol index records is a strong cooling from similar to 1.7 to 1.5 Ma. We suggest that this event reflects a reduction in Leeuwin Current intensity due to a major step in ongoing ITF constriction, accompanied by a switch from South to North Pacific source waters entering the ITF inlet. Our new data suggest that an additional ITF constriction event may have occurred in the Pleistocene.
|
|
| 7. |
- Fontorbe, Guillaume, et al.
(författare)
-
Constraints on Earth System Functioning at the Paleocene-Eocene Thermal Maximum From the Marine Silicon Cycle
- 2020
-
Ingår i: Paleoceanography and Paleoclimatology. - 2572-4517. ; 35:5
-
Tidskriftsartikel (refereegranskat)abstract
- The Paleocene-Eocene Thermal Maximum (PETM, ca. 56 Ma) is marked by a negative carbon isotope excursion (CIE) and increased global temperatures. The CIE is thought to result from the release of 13C-depleted carbon, although the source(s) of carbon and triggers for its release, its rate of release, and the mechanisms by which the Earth system recovered are all debated. Many of the proposed mechanisms for the onset and recovery phases of the PETM make testable predictions about the marine silica cycle, making silicon isotope records a promising tool to address open questions about the PETM. We analyzed silicon isotope ratios (δ30Si) in radiolarian tests and sponge spicules from the Western North Atlantic (ODP Site 1051) across the PETM. Radiolarian δ30Si decreases by 0.6‰ from a background of 1‰ coeval with the CIE, while sponge δ30Si remains consistent at 0.2‰. Using a box model to test the Si cycle response to various scenarios, we find the data are best explained by a weak silicate weathering feedback, implying the recovery was mostly driven by nondiatom organic carbon burial, the other major long-term carbon sink. We find no resolvable evidence for a volcanic trigger for carbon release, or for a change in regional oceanography. Better understanding of radiolarian Si isotope fractionation and more Si isotope records spanning the PETM are needed to confirm the global validity of these conclusions, but they highlight how the coupling between the silica and carbon cycles can be exploited to yield insight into the functioning of the Earth system.
|
|
| 8. |
- Ni, S., et al.
(författare)
-
Holocene Hydrographic Variations From the Baltic-North Sea Transitional Area (IODP Site M0059)
- 2020
-
Ingår i: Paleoceanography and Paleoclimatology. - 2572-4517. ; 35:2
-
Tidskriftsartikel (refereegranskat)abstract
- Deoxygenation affects many continental shelf seas across the world today and results in increasing areas of hypoxia (dissolved oxygen concentration ([O2]) <1.4 ml/L). The Baltic Sea is increasingly affected by deoxygenation. Deoxygenation correlates with other environmental variables such as changing water temperature and salinity and is directly linked to ongoing global climate change. To place the ongoing environmental changes into a larger context and to further understand the complex Baltic Sea history and its impact on North Atlantic climate, we investigated a high accumulation-rate brackish-marine sediment core from the Little Belt (Site M0059), Danish Straits, NW Europe, retrieved during the Integrated Ocean Drilling Program (IODP) Expedition 347. We combined benthic foraminiferal geochemistry, faunal assemblages, and pore water stable isotopes to reconstruct seawater conditions (e.g., oxygenation, temperature, and salinity) over the past 7.7 thousand years (ka). Bottom water salinity in the Little Belt reconstructed from modeled pore water oxygen isotope data increased between 7.7 and 7.5 ka BP as a consequence of the transition from freshwater to brackish-marine conditions. Salinity decreased gradually (from 30 to 24) from 4.1 to ~2.5 ka BP. By using the trace elemental composition (Mg/Ca, Mn/Ca, and Ba/Ca) and stable carbon and oxygen isotopes of foraminiferal species Elphidium selseyensis and E. clavatum, we identified that generally warming and hypoxia occurred between about 7.5 and 3.3 ka BP, approximately coinciding in time with the Holocene Thermal Maximum (HTM). These changes of bottom water conditions were coupled to the North Atlantic Oscillation (NAO) and relative sea level change.
|
|
