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- Bohm, Katja
(författare)
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Provenance of the pre-Quaternary aeolian dust deposits of East Asia
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Atmospheric mineral dust is a key component in the climate system, but its effect on and response to climate changes are poorly understood, especially over long timescales. This thesis investigates the sources of late Paleogene and Neogene dust deposits that formed under warmer-than-present global climates in the northern Chinese Loess Plateau (CLP) region in mid-latitude East Asia. Provenance research on wind-blown (aeolian) dust is one of the few available methods to constrain past wind directions, dust sources and transport pathways, which is crucial for understanding regional paleoclimate and its changes during global climate transitions.Aeolian dust deposits from three sites are analysed here: the late Paleogene (35–27 Ma) Ulantatal dust sequence, the late Neogene (7–2.6 Ma) Baode Red Clay, and a c. 4 Ma sample from the Nihewan Red Clay. A joint single-grain analysis of detrital zircon (DZ) U-Pb geochronology and detrital rutile (DR) trace element analysis was applied to trace provenance. This thesis is the first use of DR geochemistry as a provenance indicator for CLP dust, and the thesis therefore summarizes the tectonic history of potential primary source regions and presents DR data from 14 potential secondary/sedimentary dust source regions, for comparison to dust deposits.The results highlight the importance of multimineral single-grain analysis, as some provenance signals were not evident from DZ data alone. Fundamentally, the results demonstrate the long-term stability of overall atmospheric circulation in Central-East Asia, with modern-type atmospheric components existing since the late Paleogene. Dust was mostly transported by Siberian High pressure system (SH) -induced northwesterly winds, and planetary mid-latitude circulation -driven westerly winds from primary source regions of the Central Asian Orogenic Belt and Northern Tibetan Plateau, respectively. The results also show provenance changes at the Eocene-Oligocene, Miocene-Pliocene, and Pliocene-Pleistocene boundaries, indicating a long-term link between the Central-East Asian dust cycle and global cooling events. This thesis further argues that overall, both the dust provenance, and possibly the formation of northern CLP region dust deposits since the latest Eocene in general, were controlled by the Northern Hemisphere glaciations via the SH and northwesterly winds. The results shed light on the long-term environmental history of Central-East Asia and demonstrate the link between the regional dust cycle and global cooling. The findings of this thesis provide important constraints to guide future modelling work on dust-climate interactions under warmer-than-present global climates in Central-East Asia, one of the most populated regions on Earth.
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| 2. |
- Karatsolis, Boris-Theofanis
(författare)
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Late Miocene to Pliocene orbital and climatic forcing on marine productivity
- 2022
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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.
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