| 1. |
- Christensen, Beth A., et al.
(författare)
-
Late Miocene Onset of Tasman Leakage and Southern Hemisphere Supergyre Ushers in Near-Modern Circulation
- 2021
-
Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 48:18
-
Tidskriftsartikel (refereegranskat)abstract
- This study provides a Miocene-to-recent history of Tasman Leakage (TL), driving surface-to-intermediate waters from the Pacific into the Indian Ocean. TL, in addition to Indonesian ThroughFlow (ITF), constitutes an important part of the Southern Hemisphere Supergyre. Here, we employ deep-sea benthic delta C-13 timeseries from the southwestern Pacific and eastern Indian Oceans to identify the history of Tasman Leakage. The delta C-13 results combined with sedimentary evidence show that an inter-ocean connection south of Australia existed from 7 Ma onward. A southward shift in Westerlies combined with a northward movement of Australia created the oceanic corridor necessary for Tasman Leakage (between Australia and the sub-Antarctic Front) at this time. Furthermore, changes in the northern limb of the Supergyre (ITF) are evident in the sedimentary record on Broken Ridge from similar to 3 to 2 Ma when Banda Sea intermediate waters started originating from the North Pacific.
|
|
| 2. |
- Dimiza, Margarita D., et al.
(författare)
-
Coccolithophore Distribution in the Western Black Sea in the Summer of 2016
- 2023
-
Ingår i: Diversity. - : MDPI. - 1424-2818. ; 15:12
-
Tidskriftsartikel (refereegranskat)abstract
- Coccolithophores are an important component of phytoplankton abundance and biomass in the brackish environments of the Black Sea. Here, the abundance, composition, and distribution of coccolithophores were investigated in water samples taken from the first 50 m at 18 stations in the western Black Sea during a coccolithophore bloom, in June 2016. The total cell abundances ranged from 2 to 763 x 104 coccospheres L-1; Emiliania huxleyi was the most dominant species, but also Syracosphaera spp. (S. dilatata and S. molischii), Acanthoica (A. acanthifera and A. quattrospina), and Algirosphaera robusta displayed remarkably high concentrations. The formation of the seasonal thermocline significantly affects the vertical distribution of coccolithophores. Emiliania huxleyi, Syracosphaera spp., and Acanthoica spp. were restricted to the upper part of the water column, whereas high abundances of Algirosphaera robusta occurred below the thermocline. Overall, our results show significant differences in the vertical (ANOSIM R = 0.50, p = 0.0001) and spatial (ANOSIM R = 0.18, p = 0.0006) distribution of coccolithophores. Higher abundances of E. huxleyi and Syracosphaera spp. were recorded in the northwestern inner shelf region when compared to the open-sea samples. The observed coccolithophore spatial distribution is suggested to be mostly associated with the influx of less saline river water with high nutrient concentrations.
|
|
| 3. |
- 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.
|
|
| 4. |
- Karatsolis, Boris-Theofanis, et al.
(författare)
-
Abrupt conclusion of the late Miocene-early Pliocene biogenic bloom at 4.6-4.4 Ma
- 2022
-
Ingår i: Nature Communications. - : NATURE PORTFOLIO. - 2041-1723. ; 13:1
-
Tidskriftsartikel (refereegranskat)abstract
- The late Miocene-early Pliocene biogenic bloom was an extended time interval characterised by elevated ocean export productivity at numerous locations. As primary productivity is nutrient-limited at low-to-mid latitudes, this bloom has been attributed to an increase or a redistribution of available nutrients, potentially involving ocean-gateway or monsoon-related mechanisms. While the exact causal feedbacks remain debated, there is even less consensus on what caused the end of the biogenic bloom. Here, we compile Mio-Pliocene paleoproductivity proxy data from all major ocean basins to evaluate the timing and pacing of this termination. This systematic analysis reveals an abrupt and sustained reduction in low-latitude ocean productivity at 4.6–4.4 Ma. The decline in productivity coincided with a prolonged period of low orbital eccentricity and a shift towards lower-amplitude obliquity, an astronomical configuration linked to reduced East Asian Monsoon intensity and decreased riverine nutrient supply.
|
|
| 5. |
- Karatsolis, Boris-Theofanis
(författare)
-
Late Miocene to Pliocene orbital and climatic forcing on marine productivity
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The late Miocene to Pliocene was a geological time interval of global cooling, albeit in a warmer-than-present world, which is commonly used as a past analogue for future anthropogenic climate change. The investigation of marine sediments recovered by the International Ocean Discovery Program (IODP) sheds light on different paleoclimatic, paleoceanographic and paleobiological characteristics of this period. The NW Australian shelf represents an interesting area for such investigation, because it is strategically positioned near the only remaining equatorial warm-water valve on Earth. In Chapter I and Chapter II, focus is given on calcareous nannofossil time-series data and records of the element potassium (K), which is mainly delivered by rivers to the shelf, at IODP Sites U1463 and U1464. Results demonstrate that humid conditions were probably prevailing earlier than previously thought (at least since ~6 Ma), but that regional tectonics (basin subsidence) has complicated the identification of the exact onset. In addition, nannofossil assemblages data and paleotemperature gradients between the shelfal area and the eastern Indian Ocean reveal a shift in oceanographic and climatic regime that occurred between 5.4-5.2 Ma, as a likely result of an overall long-term increase in seasonality. Finally, an interval of decreasing nannofossil accumulation rates (fluxes) and a distinct change in the dominant nannoplankton species occurred between 4.6-4.4 Ma and is hypothesized to be part of broader changes in ocean nutrient availability. This hypothesis is further explored in Chapter III and Chapter IV through the investigation of a well-established period of globally elevated biogenic sedimentation (and related marine export productivity) known as the late Miocene to early Pliocene biogenic bloom. In Chapter III, age model accuracy and sample resolution of previously published biogenic sediment accumulation rate records are evaluated. The compilation of multiple records shows that an abrupt reduction in ocean paleoproductivity occurred between 4.6-4.4 Ma at (sub)tropical latitudes. This event coincided with a rather unique configuration of the Earth’s orbit, which could have led to a weakened Asian monsoon activity and therefore reduced river runoff and nutrient supply to the ocean. Chapter IV focuses on the comparison between the calcareous nannofossil assemblages at the NW Australian shelf sites and ODP Site 1264 in the South Atlantic Ocean, across the termination of the biogenic bloom. Although the overall decrease in paleoproductivity occurred around the same time, the shift in species dominance across the end of the biogenic bloom, as shown in the tropical Indian Ocean, is not observed at ODP Site 1264.
|
|
| 6. |
- Karatsolis, Boris-Theofanis, et al.
(författare)
-
Late Neogene nannofossil assemblages as tracers of ocean circulation and paleoproductivity over the NW Australian shelf
- 2023
-
Ingår i: Climate of the Past Discussions. - : European Geosciences Union (EGU). - 1814-9340 .- 1814-9359. ; 19, s. 765-786
-
Tidskriftsartikel (refereegranskat)abstract
- Late Miocene to Pliocene sediments from the NW Australian shelf provide unique records of paleoclimatic variations under warmer-than-present conditions. During the period from 6–3.5 million years ago (Ma), the area was dominated by warm, tropical waters supplied by an intensified, uninterrupted Indonesian Throughflow and characterised by prevailing humid conditions and increased precipitation. Despite the available information regarding the general paleoclimatic conditions, little is known about the concurrent regional ocean circulation patterns and the relative intensity of seasonally flowing boundary currents, such as the Leeuwin Current. Recent investigations of sediments recovered from the shelf during International Ocean Discovery Program (IODP) Expedition 356, have produced a number of well-dated time-series that cover the late Neogene and can therefore assist with more detailed reconstructions. In this study, we investigate two astronomically-tuned calcareous nannofossil records from IODP Sites U1463 and U1464 that can help us trace and understand long-term changes in ocean circulation and seasonality, as well as their effect on nutrient replenishment in the upper photic zone. By looking into shifts in the dominant species within the nannofossil assemblage, and comparing them with paleotemperature gradients between the NW Australian shelf and the eastern Indian Ocean, we identify the main periods of change in stratification and nutrient availability that occurred over the continental shelf. Our results indicate a significant change in ecological and oceanographic regime that occurred across the Miocene to Pliocene boundary (5.4–5.2 Ma), which can be attributed to an increase in seasonality and an overall intensification of the upper water column mixing over the shelf. Major changes in the nannofossil assemblages that reflect broader-scale processes and evolutionary events, such as the extinction of Sphenolithus spp. (~3.54 Ma) and the termination of the late Miocene to early Pliocene biogenic bloom in the eastern Indian Ocean (4.6–4.4 Ma), occurred long after this regional regime shift.
|
|
| 7. |
- Karatsolis, Boris-Theofanis, 1992-
(författare)
-
Latest Miocene – Early Pliocene Paleoclimate and Phytoplankton Productivity
- 2020
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Paper IPre-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. Post-expedition 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 6.1-4 million years ago (Ma). Overall, the investigated proxies show high correlation between both sites, except during 6.1-5.7 Ma. During this latest Miocene 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 ~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 high K wt% and coccolith abundances at Site U1464 indicate that humidity was already enhanced since at least 6 Ma. This age is consistent with data supporting a southward movement of the Intertropical Convergence Zone rain belt at ~7 Ma.Paper IIThe early Pliocene was a warm period with increased ocean primary productivity, as part of a global paleoceanographic event called the late Miocene-early Pliocene biogenic bloom (~9-3.5 Ma). Many tectonic and paleoclimatic mechanisms, mainly linked to an increase and redistribution of nutrient supply in the ocean, have been proposed as driving forces for this event. However, the main phase of increased productivity and the termination of this event appear to be diachronous in different ocean basins. Here, we compiled proxy data for early Pliocene paleoproductivity from all major ocean basins, including both calcareous and siliceous plankton groups. After re-evaluating the age model resolution of the available paleo-records, we demonstrate that a main stage of decrease in primary productivity occurred during ~4.6-4.4 Ma. We then show that this productivity collapse coincided with an orbital configuration of long-term reduction in eccentricity amplitude and low amplitude obliquity. This combination of orbital parameters could have significantly affected seasonality and nutrient availability in the global ocean and suggests a previously undescribed paleoclimatic forcing that may have been a crucial step in contributing to the end of the biogenic bloom
|
|
| 8. |
- Karatsolis, Boris-Theofanis, et al.
(författare)
-
Nannofossil assemblages across the late Miocene to early Pliocene biogenic bloom termination: South Atlantic vs. Indian Ocean signatures
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The late Miocene to early Pliocene biogenic bloom was an interval of increased biogenic sediment burial documented in all major oceans, between ~9-3.5 Ma. Accumulation rates of carbonates and opal, produced by phytoplanktonic organisms such as calcareous nannoplankton and diatoms, have been widely used to understand when and why this interval occurred and how long it lasted. However, information is still limited on the synchronicity of the termination of the biogenic bloom across ocean basins, although some recent studies have started to tackle this question. Drury and others (2021) demonstrated that the biogenic bloom in the midlatitude South Atlantic (ODP Site 1264) ended at ~3.3 Ma, whereas Karatsolis and others (2022) argued for a synchronous termination of the biogenic bloom in low latitudes at 4.6-4.4 Ma. Notably, this synchronous decrease in paleoproductivity was also accompanied by significant restructuring of the calcareous nannofossil assemblages, at least offshore NW Australia (IODP Site U1463). Since calcareous nannoplankton are the main drivers of change in carbonate burial during the Neogene, such shifts could be used as a biostratigraphic tool to help identify the end of the biogenic bloom across ocean basins. In this study, we investigate the changes in carbonate fluxes relative to the biostratigraphy at ODP Site 1264 and compare the calcareous nannofossil assemblages to those observed at IODP Site U1463. This comparison reveals that although the decrease in carbonate fluxes at ODP Site 1264 might have occurred in good accordance with the proposed end of the biogenic bloom in low latitudes, it was not accompanied by a change in the dominant calcareous nannofossil species, as was the case in the Indian Ocean. This differential expression could be attributed to ocean circulation patterns and different paleoceanographic regimes between the two areas.
|
|
| 9. |
- Karatsolis, Boris-Theofanis, 1992-, et al.
(författare)
-
Ocean productivity response to orbital forcing during the early Pliocene
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The early Pliocene was a warm period with increased ocean primary productivity, as part of a global paleoceanographic event called the late Miocene-early Pliocene biogenic bloom (~9-3.5 Ma). Many tectonic and paleoclimatic mechanisms, mainly linked to an increase and redistribution of nutrient supply in the ocean, have been proposed as driving forces for this event. However, the main phase of increased productivity and the termination of this event appear to be diachronous in different ocean basins. Here, we compiled proxy data for early Pliocene paleoproductivity from all major ocean basins, including both calcareous and siliceous plankton groups. After re-evaluating the age model resolution of the available paleo-records, we demonstrate that a main stage of decrease in primary productivity occurred during ~4.6-4.4 Ma. We then show that this productivity collapse coincided with an orbital configuration of long-term reduction in eccentricity amplitude and low amplitude obliquity. This combination of orbital parameters could have significantly affected seasonality and nutrient availability in the global ocean and suggests a previously undescribed paleoclimatic forcing that may have been a crucial step in contributing to the end of the biogenic bloom.
|
|
| 10. |
- 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.
|
|
