Planktic foraminiferal proxy development and application to paleoceanographic change in the Southwest Pacific Ocean

<p>This thesis investigates the use of foraminiferal calcite geochemical and physical properties as paleoceanographic proxies, to improve identification of past climatic change and provide a more quantitative basis for forecasts of future climate. I have developed and used these proxies on a high resolution, well-dated marine sediment core, MD97 2121 from north of the Subtropical Front (STF) off the eastern central North Island of New Zealand to determine paleoceanographic changes in the South Pacific Gyre since the last glacial period, 25 ka to present. Various analytical methods to measure foraminiferal calcite trace element geochemistry were first investigated using core top samples. Two main analytical techniques were deployed; “pseudo” solution- or laser ablation-based ICPMS analysis. Ratios tested include Mg/Ca, Sr/Ca, Ba/Ca, Zn/Ca, Mn/Ca and Al/Ca. Trace element/calcium ratios Mg/Ca and Sr/Ca values were consistent between these methods, provided that currently recommended ‘Mg-cleaning’ protocols were followed for solution-based measurements. However, discrepancies of up to an order-of-magnitude for Zn/Ca, Mn/Ca and Ba/Ca occurred between solution and laser ablation-based measurements if both oxidative and reductive cleaning techniques were not employed prior to solution-based analysis. Using down-core trace element values Mg/Ca, Zn/Ca, Mn/Ca and Ba/Ca from MD97 2121, coupled with modern core top and plankton-tow samples, multiple geochemical proxies for the SW Pacific Ocean were developed and/or tested. Results suggest that Zn/Ca may act as (i) a surface water mass tracer, in this case differentiating between subtropical and subantarctic surface waters and (ii) a proxy for nutrients. Mg/Ca and Zn/Ca values from different test chambers in Globigerina bulloides were also found to reliably re-construct surface ocean temperature and nutrient stratification. Using these new proxies, coupled with oxygen isotopes, standard Mg/Ca paleothermometry and foraminiferal assemblage data, I show that surface water nutrient and thermal stratification significantly reduced during the last glacial period. In addition, the relative strength of the South Pacific Gyre, which affects the inflow of subtropical water to New Zealand, was a major influence during the last glacial termination. In particular, the period from 17-14.5 ka, otherwise known as the ‘Mystery Interval’, appears to be genuinely anomalous with foraminifera indicating cooling trends while alkenones continue to warm. This may reflect changes to both gyre strength and Antarctic forcing prior to the Antarctic Cold Reversal (14.2-12.5ka) and an offset in the timing of species productivity. The high resolution Mg/Ca paleotemperature record developed here, together with published alkenone paleotemperatures were compared to core MD97 2120, south of the STF to evaluate the relationship between Mg/Ca and alkenones temperatures and how these reflect environmental change. It appears that the season of maximum alkenone and G. bulloides flux varied over the last 25kyr in response to insolation and water mass changes. During the glacial period north and south of the STF alkenone seasonal flux was summer dominated. However, during the Holocene while seasonal alkenone flux remained summer or annual dominated in the north, it shifted to a spring productivity cycle south of the STF. The foraminifera G. bulloides glacial period flux was likely have been spring dominated both north and south of the STF, maintaining a spring bloom cycle south of the STF, while shifting to a summer or annual cycle to the north during the Holocene. These seasonal offsets may have acted to dampen or exacerbate the glacial-Holocene temperature offsets by up to 4°C especially for the surface dwelling, alkenone producing coccolithophores. Seasonality changes of the coccolithophore and foraminifera make direct comparison of alkenone and Mg/Ca G. bulloides paleothermometers challenging. However, despite the complexity, offsets in the paleotemperatures may help to elucidate changes in the paleoceanography. The use of G. bulloides size normalised weight (SNW) as a proxy for surface water carbonate ion concentration ([CO₃⁼]) was investigated by comparing modern SNW data sets from five different ocean regions to their specific environmental variables including [CO₃⁼], chlorophyll-a, nutrient and temperature values. It was identified that the ‘ocean’ from which the foraminifera originated appeared to have the strongest control over shell SNW, potentially reflecting geographically distinct, genetic variations within the G. bulloides species. Within ‘ocean’ regions no consistent environmental variable(s) could be identified that appeared to control shell SNW in all regions. From the 25 ka to present, shell SNWs from the SW Pacific Ocean were compared to the North Atlantic and were found to be heavier during the glacial period regardless of ocean region. This may reflect multiple factors including increased surface ocean CO₃⁼, possibly combined with changes in primary productivity. Calcification of G. bulloides tests appears to be region specific; therefore, proxy calibrations based on shell SNW for one ocean are not applicable to other settings.</p>

Planktic foraminiferal proxy development and application to paleoceanographic change in the Southwest Pacific Ocean

<p>This thesis investigates the use of foraminiferal calcite geochemical and physical properties as paleoceanographic proxies, to improve identification of past climatic change and provide a more quantitative basis for forecasts of future climate. I have developed and used these proxies on a high resolution, well-dated marine sediment core, MD97 2121 from north of the Subtropical Front (STF) off the eastern central North Island of New Zealand to determine paleoceanographic changes in the South Pacific Gyre since the last glacial period, 25 ka to present. Various analytical methods to measure foraminiferal calcite trace element geochemistry were first investigated using core top samples. Two main analytical techniques were deployed; “pseudo” solution- or laser ablation-based ICPMS analysis. Ratios tested include Mg/Ca, Sr/Ca, Ba/Ca, Zn/Ca, Mn/Ca and Al/Ca. Trace element/calcium ratios Mg/Ca and Sr/Ca values were consistent between these methods, provided that currently recommended ‘Mg-cleaning’ protocols were followed for solution-based measurements. However, discrepancies of up to an order-of-magnitude for Zn/Ca, Mn/Ca and Ba/Ca occurred between solution and laser ablation-based measurements if both oxidative and reductive cleaning techniques were not employed prior to solution-based analysis. Using down-core trace element values Mg/Ca, Zn/Ca, Mn/Ca and Ba/Ca from MD97 2121, coupled with modern core top and plankton-tow samples, multiple geochemical proxies for the SW Pacific Ocean were developed and/or tested. Results suggest that Zn/Ca may act as (i) a surface water mass tracer, in this case differentiating between subtropical and subantarctic surface waters and (ii) a proxy for nutrients. Mg/Ca and Zn/Ca values from different test chambers in Globigerina bulloides were also found to reliably re-construct surface ocean temperature and nutrient stratification. Using these new proxies, coupled with oxygen isotopes, standard Mg/Ca paleothermometry and foraminiferal assemblage data, I show that surface water nutrient and thermal stratification significantly reduced during the last glacial period. In addition, the relative strength of the South Pacific Gyre, which affects the inflow of subtropical water to New Zealand, was a major influence during the last glacial termination. In particular, the period from 17-14.5 ka, otherwise known as the ‘Mystery Interval’, appears to be genuinely anomalous with foraminifera indicating cooling trends while alkenones continue to warm. This may reflect changes to both gyre strength and Antarctic forcing prior to the Antarctic Cold Reversal (14.2-12.5ka) and an offset in the timing of species productivity. The high resolution Mg/Ca paleotemperature record developed here, together with published alkenone paleotemperatures were compared to core MD97 2120, south of the STF to evaluate the relationship between Mg/Ca and alkenones temperatures and how these reflect environmental change. It appears that the season of maximum alkenone and G. bulloides flux varied over the last 25kyr in response to insolation and water mass changes. During the glacial period north and south of the STF alkenone seasonal flux was summer dominated. However, during the Holocene while seasonal alkenone flux remained summer or annual dominated in the north, it shifted to a spring productivity cycle south of the STF. The foraminifera G. bulloides glacial period flux was likely have been spring dominated both north and south of the STF, maintaining a spring bloom cycle south of the STF, while shifting to a summer or annual cycle to the north during the Holocene. These seasonal offsets may have acted to dampen or exacerbate the glacial-Holocene temperature offsets by up to 4°C especially for the surface dwelling, alkenone producing coccolithophores. Seasonality changes of the coccolithophore and foraminifera make direct comparison of alkenone and Mg/Ca G. bulloides paleothermometers challenging. However, despite the complexity, offsets in the paleotemperatures may help to elucidate changes in the paleoceanography. The use of G. bulloides size normalised weight (SNW) as a proxy for surface water carbonate ion concentration ([CO₃⁼]) was investigated by comparing modern SNW data sets from five different ocean regions to their specific environmental variables including [CO₃⁼], chlorophyll-a, nutrient and temperature values. It was identified that the ‘ocean’ from which the foraminifera originated appeared to have the strongest control over shell SNW, potentially reflecting geographically distinct, genetic variations within the G. bulloides species. Within ‘ocean’ regions no consistent environmental variable(s) could be identified that appeared to control shell SNW in all regions. From the 25 ka to present, shell SNWs from the SW Pacific Ocean were compared to the North Atlantic and were found to be heavier during the glacial period regardless of ocean region. This may reflect multiple factors including increased surface ocean CO₃⁼, possibly combined with changes in primary productivity. Calcification of G. bulloides tests appears to be region specific; therefore, proxy calibrations based on shell SNW for one ocean are not applicable to other settings.</p>

Redefining the Subsurface Biosphere: Characterization of Fungi Isolated From Energy-Limited Marine Deep Subsurface Sediment

The characterization of metabolically active fungal isolates within the deep marine subsurface will alter current ecosystem models and living biomass estimates that are limited to bacterial and archaeal populations. Although marine fungi have been studied for over fifty years, a detailed description of fungal populations within the deep subsurface is lacking. Fungi possess metabolic pathways capable of utilizing previously considered non-bioavailable energy reserves. Therefore, metabolically active fungi would occupy a unique niche within subsurface ecosystems, with the potential to provide an organic carbon source for heterotrophic prokaryotic populations from the transformation of non-bioavailable energy into substrates, as well as from the fungal necromass itself. These organic carbon sources are not currently being considered in subsurface energy budgets. Sediments from South Pacific Gyre subsurface, one of the most energy-limited environments on Earth, were collected during the Integrated Ocean Drilling Program Expedition 329. Anoxic and oxic sediment slurry enrichments using fresh sediment were used to isolate multiple fungal strains in media types that varied in organic carbon substrates and concentration. Metabolically active and dormant fungal populations were also determined from nucleic acids extracted from in situ cryopreserved South Pacific Gyre sediments. For further characterization of physical growth parameters, two isolates were chosen based on their representation of the whole South Pacific Gyre fungal community. Results from this study show that fungi have adapted to be metabolically active and key community members in South Pacific Gyre sediments and potentially within global biogeochemical cycles.

BOMB-PRODUCED RADIOCARBON ACROSS THE SOUTH PACIFIC GYRE—A NEW RECORD FROM AMERICAN SAMOA WITH UTILITY FOR FISHERIES SCIENCE

ABSTRACT Coral skeletal structures can provide a robust record of nuclear bomb produced 14C with valuable insight into air-sea exchange processes and water movement with applications to fisheries science. To expand these records in the South Pacific, a coral core from Tutuila Island, American Samoa was dated with density band counting covering a 59-yr period (1953–2012). Seasonal signals in elemental ratios (Sr/Ca and Ba/Ca) and stable carbon (δ13C) values across the coral core corroborated the well-defined annual band structure and highlighted an ocean climate shift from the 1997–1998 El Niño. The American Samoa coral 14C measurements were consistent with other regional records but included some notable differences across the South Pacific Gyre (SPG) at Fiji, Rarotonga, and Easter Island that can be attributed to decadal ocean climate cycles, surface residence times and proximity to the South Equatorial Current. An analysis of the post-peak 14C decline associated with each coral record indicated 14C levels are beginning to merge for the SPG. This observation, coupled with otolith measurements from American Samoa, reinforces the perspective that bomb 14C dating can be performed on fishes and other marine organisms of the region using the post-peak 14C decline to properly inform fisheries management in the South Pacific.

Seasonal dynamics of phytoplankton and bacterioplankton at the ultra-oligotrophic southeastern Mediterranean Sea

The Eastern Mediterranean Sea (EMS) is an ultra-oligotrophic, enclosed basin strongly impacted by regional and global anthropogenic stressors. Here, we describe the annual (2018-19) dynamics of phyto- and bacterioplankton (abundances, pigments and productivity) in relation to the physical and chemical conditions in the photic water column at the offshore EMS water (Station THEMO-2, ~1,500m depth, 50km offshore). Annual patterns in phytoplankton biomass (as chlorophyll a), primary and bacterial productivity differed between the mixed winter (January-April) and the thermally stratified (May-December) periods. Prochlorococcus and Synechococcus numerically dominated the picophytoplankton populations, with each clade revealing different temporal and depth patterns, while pico-eukaryotes (primarily haptophytes) were less abundant, yet likely contributed significant biomass. Integrated primary productivity (~32 gC m-2 y-1) was lower compared with other well-studied oligotrophic locations, including the north Atlantic and Pacific (HOT and BATS observatories), the western Mediterranean (DYFAMED observatory) and the Red Sea, and was on-par with the ultra-oligotrophic South Pacific Gyre. In contrast, integrated bacterial production (~11 gC m-2 y-1) was similar to other oligotrophic locations. Phytoplankton seasonal dynamics were reminiscent of those at BATS and the Red Sea, suggesting an observable effect of winter mixing in this ultra-oligotrophic location. These results highlight the ultra-oligotrophic conditions in the EMS and provide, for the first time in this region, a full-year baseline and context to ocean observatories in the region.

Extensive Microbial Processing of Polysaccharides in the South Pacific Gyre via Selfish Uptake and Extracellular Hydrolysis

Primary productivity occurs throughout the deep euphotic zone of the oligotrophic South Pacific Gyre (SPG), fueled largely by the regeneration of nutrients and thus recycling of organic matter. We investigated the heterotrophic capabilities of the SPG’s bacterial communities by examining their ability to process polysaccharides, an important component of marine organic matter. We focused on the initial step of organic matter degradation by measuring the activities of extracellular enzymes that hydrolyze six different polysaccharides to smaller sizes. This process can occur by two distinct mechanisms: “selfish uptake,” in which initial hydrolysis is coupled to transport of large polysaccharide fragments into the periplasmic space of bacteria, with little to no loss of hydrolysis products to the external environment, and “external hydrolysis,” in which low molecular weight (LMW) hydrolysis products are produced in the external environment. Given the oligotrophic nature of the SPG, we did not expect high enzymatic activity; however, we found that all six polysaccharides were hydrolyzed externally and taken up selfishly in the central SPG, observations that may be linked to a comparatively high abundance of diatoms at the depth and location sampled (75 m). At the edge of the gyre and close to the center of the gyre, four of six polysaccharides were externally hydrolyzed, and a lower fraction of the bacterial community showed selfish uptake. One polysaccharide (fucoidan) was selfishly taken up without measurable external hydrolysis at two stations. Additional incubations of central gyre water from depths of 1,250 and 2,800 m with laminarin (an abundant polysaccharide in the ocean) led to extreme growth of opportunistic bacteria (Alteromonas), as tracked by cell counts and next generation sequencing of the bacterial communities. These Alteromonas appear to concurrently selfishly take up laminarin and release LMW hydrolysis products. Overall, extracellular enzyme activities in the SPG were similar to activities in non-oligotrophic regions, and a considerable fraction of the community was capable of selfish uptake at all three stations. A diverse set of bacteria responded to and are potentially important for the recycling of organic matter in the SPG.