Reconstructing oceanographic conditions through the deglacial and holocene (ca. 15 ka) at the sub-antarctic island of South Georgia

South Georgia is located in the path of the Antarctic Circumpolar Current (ACC) and the Southern Hemisphere Westerly Winds (SWW), south of the Antarctic Polar Front (APF) and surrounded by an anticyclonic loop of the Southern ACC Front (SACCF). The waters surrounding South Georgia are highly productive and variability in the interaction of the SACCF with the island may impact productivity on the shelf. Of wider significance is the large phytoplankton bloom that occurs to the north-west of South Georgia, where the SACCF retroflects to return to its cyclonic circumpolar circulation. This bloom represents one of the largest seasonal sinks of atmospheric CO2 in the Southern Ocean. Given the likely importance of the Southern Ocean as a carbon sink/source during the transition between glacial and interglacial periods a better understanding of the bloom dynamics could be important to our assessment of the Southern Ocean’s role in these transitions. This thesis investigates deglacial and Holocene variability in productivity and sea-surface temperature on the north-eastern shelf of South Georgia, through the development of the first palaeoceanographic records from the island. To support the palaeoceanographic interpretation a comprehensive taxonomic study of the benthic foraminifera of the South Georgia shelf was developed, and the importance of foraminiferal ecology to test carbon isotope composition was investigated. The taxonomic assessment of the shelf benthic foraminifera resulted in the description of 78 species, assigned to 57 genera from 34 families (of these 58 species are identified to species level, while 20 species are discussed in open nomenclature). The investigation of foraminiferal ecology highlighted the importance of understanding the ecology of the species being utilised for isotope analysis to the application of this data to palaeoceanographic interpretations. In addition, a novel calcification strategy was proposed, in which a most benthic foraminiferal calcification occurs at the sediment-seawater interface. The palaeoceanographic records presented here support the hypothesis that the ice sheet on South Georgia during the Last Glacial Maximum extended onto the shelf. Potential evidence for the formation of a calving bay re-entrant over Royal Bay trough indicates that grounded ice may have been present on the shallower shelf until ca. 15 cal. kyr BP, having retreated rapidly over the deeper cross shelf troughs after the Last Glacial Maximum. Evidence is also found for the influence of the Antarctic Cold Reversal at South Georgia. During the Holocene, I propose that both sea-surface temperature (SST) and shelf productivity may have been driven by variation in the SWW, except during the early Holocene when sea-ice proximity may play an important role in productivity variability. Changes in the intensity of the SWW at South Georgia drive SST through their impact of local foehn wind frequency and shelf productivity through variation in coastal upwelling. This interpretation of the variability in shelf SST and productivity indicates that the SWW were more intense at South Georgia during the mid-Holocene, and weaker during the early and late Holocene. Similar patterns of SWW variability have been reported from other locations at the same latitude, although some records have been interpreted to reveal different patterns of SWW change. Variation in SWW intensity at South Georgia may be due to either latitudinal migration of the SWW, or to expansion and contraction of the core SWW band, both of which are linked to Southern Hemisphere temperature change

"Live” (stained) benthic foraminiferal living depths, stable isotopes, and taxonomy offshore South Georgia, Southern Ocean: implications for calcification depths

It is widely held that benthic foraminifera exhibit species-specific calcification depth preferences, with their tests recording sediment pore water chemistry at that depth (i.e. stable isotope and trace metal compositions). This assumed depth habitat-specific pore water chemistry relationship has been used to reconstruct various palaeoenvironmental parameters, such as bottom water oxygenation. However, many deep-water foraminiferal studies show wide intra-species variation in sediment living depth but relatively narrow intra-species variation in stable isotope composition. To investigate this depth habitat- stable isotope relationship on the shelf we analysed depth distribution and stable isotopes of “living” (Rose Bengal stained) benthic foraminifera from two box cores collected on the South Georgia shelf (ranging from 250–300 m water depth). We provide a comprehensive taxonomic analysis of the benthic fauna, comprising 79 taxonomic groupings. The fauna shows close affinities with shelf assemblages from around Antarctica. We find “live” specimens of a number calcareous species from a range of depths in the sediment column. Stable isotope ratios (δ13C and δ18O) were measured on stained specimens of three species, Astrononion echolsi, Cassidulinoides porrectus and Buccella sp. 1, at 1 cm depth intervals within the down-core sediment sequences. In agreement with studies in deep water settings, we find no significant intraspecies variability in either δ13C foram or δ18O foram with sediment living depth on the South Georgia shelf. Our findings add to the growing evidence that infaunal benthic foraminiferal species calcify at a fixed depth. Given the wide range of depths that we find “living” ‘infaunal’ species, we speculate that they may actually calcify predominantly at the sediment-seawater interface, where carbonate ion concentration and organic carbon availability is at a maximum

South Georgia marine productivity over the past 15 ka and implications for glacial evolution

The subantarctic islands of South Georgia are located in the Southern Ocean, and they may be sensitive to future climate warming. However, due to a lack of well-dated subantarctic palaeoclimate archives, there is still uncertainty about South Georgia’s response to past climate change. Here, we reconstruct primary productivity changes and infer Holocene glacial evolution by analysing two marine gravity cores: one near Cumberland Bay on the inner South Georgia shelf (GC673: ca. 9.5 to 0.3cal.kyrBP) and one offshore of Royal Bay on the mid-shelf (GC666: ca. 15.2cal.kyrBP to present). We identify three distinct benthic foraminiferal assemblages characterised by the dominance of Miliammina earlandi, Fursenkoina fusiformis, and Cassidulinoides parkerianus that are considered alongside foraminiferal stable isotopes and the organic carbon and biogenic silica accumulation rates of the host sediment. The M. earlandi assemblage is prevalent during intervals of dissolution in GC666 and reduced productivity in GC673. The F. fusiformis assemblage coincides with enhanced productivity in both cores. Our multiproxy analysis provides evidence that the latest Pleistocene to earliest Holocene (ca. 15.2 to 10.5cal.kyrBP) was a period of high productivity associated with increased glacial meltwater discharge. The mid–late Holocene (ca. 8 to 1cal.kyrBP), coinciding with a fall in sedimentation rates and lower productivity, was likely a period of reduced glacial extent but with several short-lived episodes of increased productivity from minor glacial readvances. The latest Holocene (from ca. 1cal.kyrBP) saw an increase in productivity and glacial advance associated with cooling temperatures and increased precipitation which may have been influenced by changes in the southwesterly winds over South Georgia. We interpret the elevated relative abundance of F. fusiformis as a proxy for increased primary productivity which, at proximal site GC673, was forced by terrestrial runoff associated with the spring–summer melting of glaciers in Cumberland Bay. Our study refines the glacial history of South Georgia and provides a more complete record of mid–late Holocene glacial readvances with robust chronology. Our results suggest that South Georgia glaciers were sensitive to modest climate changes within the Holocene

​​South Georgia marine productivity over the past 15 ka and implications for glacial evolution​

The subantarctic islands of South Georgia are located in the Southern Ocean, and they may be sensitive to future climate warming. However, due to a lack of well-dated subantarctic palaeoclimate archives, there is still uncertainty about South Georgia's response to past climate change. Here, we reconstruct primary productivity changes and infer Holocene glacial evolution by analysing two marine gravity cores: one near Cumberland Bay on the inner South Georgia shelf (GC673: ca. 9.5 to 0.3 cal. kyr BP) and one offshore of Royal Bay on the mid-shelf (GC666: ca. 15.2 cal. kyr BP to present). We identify three distinct benthic foraminiferal assemblages characterised by the dominance of Miliammina earlandi, Fursenkoina fusiformis, and Cassidulinoides parkerianus that are considered alongside foraminiferal stable isotopes and the organic carbon and biogenic silica accumulation rates of the host sediment. The M. earlandi assemblage is prevalent during intervals of dissolution in GC666 and reduced productivity in GC673. The F. fusiformis assemblage coincides with enhanced productivity in both cores. Our multiproxy analysis provides evidence that the latest Pleistocene to earliest Holocene (ca. 15.2 to 10.5 cal. kyr BP) was a period of high productivity associated with increased glacial meltwater discharge. The mid–late Holocene (ca. 8 to 1 cal. kyr BP), coinciding with a fall in sedimentation rates and lower productivity, was likely a period of reduced glacial extent but with several short-lived episodes of increased productivity from minor glacial readvances. The latest Holocene (from ca. 1 cal. kyr BP) saw an increase in productivity and glacial advance associated with cooling temperatures and increased precipitation which may have been influenced by changes in the southwesterly winds over South Georgia. We interpret the elevated relative abundance of F. fusiformis as a proxy for increased primary productivity which, at proximal site GC673, was forced by terrestrial runoff associated with the spring–summer melting of glaciers in Cumberland Bay. Our study refines the glacial history of South Georgia and provides a more complete record of mid–late Holocene glacial readvances with robust chronology. Our results suggest that South Georgia glaciers were sensitive to modest climate changes within the Holocene

Reconstructing oceanographic conditions through the deglacial and holocene (ca. 15 ka) at the sub-antarctic island of South Georgia

South Georgia is located in the path of the Antarctic Circumpolar Current (ACC) and the Southern Hemisphere Westerly Winds (SWW), south of the Antarctic Polar Front (APF) and surrounded by an anticyclonic loop of the Southern ACC Front (SACCF). The waters surrounding South Georgia are highly productive and variability in the interaction of the SACCF with the island may impact productivity on the shelf. Of wider significance is the large phytoplankton bloom that occurs to the north-west of South Georgia, where the SACCF retroflects to return to its cyclonic circumpolar circulation. This bloom represents one of the largest seasonal sinks of atmospheric CO2 in the Southern Ocean. Given the likely importance of the Southern Ocean as a carbon sink/source during the transition between glacial and interglacial periods a better understanding of the bloom dynamics could be important to our assessment of the Southern Ocean’s role in these transitions. This thesis investigates deglacial and Holocene variability in productivity and sea-surface temperature on the north-eastern shelf of South Georgia, through the development of the first palaeoceanographic records from the island. To support the palaeoceanographic interpretation a comprehensive taxonomic study of the benthic foraminifera of the South Georgia shelf was developed, and the importance of foraminiferal ecology to test carbon isotope composition was investigated. The taxonomic assessment of the shelf benthic foraminifera resulted in the description of 78 species, assigned to 57 genera from 34 families (of these 58 species are identified to species level, while 20 species are discussed in open nomenclature). The investigation of foraminiferal ecology highlighted the importance of understanding the ecology of the species being utilised for isotope analysis to the application of this data to palaeoceanographic interpretations. In addition, a novel calcification strategy was proposed, in which a most benthic foraminiferal calcification occurs at the sediment-seawater interface. The palaeoceanographic records presented here support the hypothesis that the ice sheet on South Georgia during the Last Glacial Maximum extended onto the shelf. Potential evidence for the formation of a calving bay re-entrant over Royal Bay trough indicates that grounded ice may have been present on the shallower shelf until ca. 15 cal. kyr BP, having retreated rapidly over the deeper cross shelf troughs after the Last Glacial Maximum. Evidence is also found for the influence of the Antarctic Cold Reversal at South Georgia. During the Holocene, I propose that both sea-surface temperature (SST) and shelf productivity may have been driven by variation in the SWW, except during the early Holocene when sea-ice proximity may play an important role in productivity variability. Changes in the intensity of the SWW at South Georgia drive SST through their impact of local foehn wind frequency and shelf productivity through variation in coastal upwelling. This interpretation of the variability in shelf SST and productivity indicates that the SWW were more intense at South Georgia during the mid-Holocene, and weaker during the early and late Holocene. Similar patterns of SWW variability have been reported from other locations at the same latitude, although some records have been interpreted to reveal different patterns of SWW change. Variation in SWW intensity at South Georgia may be due to either latitudinal migration of the SWW, or to expansion and contraction of the core SWW band, both of which are linked to Southern Hemisphere temperature change