Sources and biogeochemistry of bio-active trace metals in the Southern Ocean and coastal Antarctica
Tian, H.-A. (2024). Sources and biogeochemistry of bio-active trace metals in the Southern Ocean and coastal Antarctica. PhD Thesis. [S.n.]: Utrecht. ISBN 978-90-6266-682-9. 1-286 pp.
|  |
| Abstract |
Bio-essential trace elements, such as iron (Fe) and zinc (Zn), also known as micronutrients, are critical for all life because of their biochemical roles in various metabolic processes. They are also pivotal for marine phytoplankton growth, the base of the marine food web. For instance, Fe participates in vital functions such as photosynthesis, respiration, and nutrient uptake, while Zn is present in nearly 300 enzymes and is utilized in common enzymes such as carbonic anhydrase (CA) and alkaline phosphatase (AP) for CO2 hydration and organic phosphorus uptake, respectively. Moreover, other trace elements are not strictly bio-essential, for example, cadmium (Cd), has been shown to replace Zn in CA in marine diatoms when Zn becomes unavailable. As a result, the availability of these trace elements in oceans significantly impacts marine ecosystems and, hence, global climate. The availability of trace elements in oceans is influenced by external sources, chemical speciation, and biogeochemical processes such as biological uptake, remineralization, and scavenging, which regulate primary productivity in the surface ocean. However, dissolved concentrations of these elements are often low in the surface ocean where they are needed for primary productivity, making sampling and accurate analysis a challenge. In the oceans, High-Nutrient Low-Chlorophyll (HNLC) surface regions are characterized by sufficient macronutrient (e.g., phosphate, nitrite/nitrate, sillicate) supply but limited primary productivity due to the lack of Fe and light. As the largest HNLC region, the Southern Ocean is suggested to have a great potential to absorb a large amount of atmospheric CO2 if the phytoplankton requirements for Fe and light are fulfilled. For example, in coastal Antarctica, long-lasting phytoplankton blooms over spring and summer have been observed and attributed to increasing external Fe input, such as upwelling of deep water, continental sediments, and sea ice- or glacier-associated Fe. In addition to the exchange of atmospheric CO2, the Southern Ocean plays a vital role in the redistribution of heat and freshwater in the global climate system. For instance, dense water formation driven by extensive sea ice production and brine rejection during winter in coastal Antarctic regions contributes to the formation of Antarctic Bottom Water (AABW) that circulates northward into multiple ocean basins and facilitates advection of water masses between high and low latitude oceans. Overall, given the importance of the Southern Ocean to the global climate and ocean circulation, it is crucial to understand its marine biogeochemistry, notably the cycling of trace elements in coastal Antarctic regions, and their influence on globally relevant processes. In the past two decades, the international GEOTRACES program has explored the biogeochemical cycles and oceanic distributions of trace elements and their isotopes in oceans. The combination of concurrent concentration and isotopic composition analyses of trace elements has greatly helped us to disentangle complex biogeochemical processes and identify different external sources. However, compared to the Pacific, Atlantic, Indian, and Arctic Oceans, dissolved trace element data in the Southern Ocean, especially around coastal Antarctica, are relatively scarce owing to limited sampling campaigns due to harsh weather conditions and poor accessibility related to extensive sea ice coverage in coastal Antarctica, as well as the requirement of relatively large water volumes needed to determine the extremely low concentrations (e.g., Fe) in the surface Southern Ocean. The scarcity of data has not only hindered our understanding of global nutrient cycling and how changes in coastal Antarctica (e.g., ice melting) impact nutrient supply, but also limited our computational ability (e.g., biogeochemical models) to accurately predict the changes in biogeochemistry and cycling of trace elements in the future at a global and long-term scale. In this thesis, two bio-essential trace elements, Fe and Zn, together with Cd, and their isotopic compositions, are studied in two distinct coastal Antarctic regions – the Amundsen Sea (AS) and the Weddell Sea (WS). The AS is characterized by the extensive intrusion of warm modified Circumpolar Deep Water (mCDW) onto the continental shelf through glacial troughs that lead to rapid ice sheet melting as it flows under ice shelves (i.e., Dotson Ice Shelf, DIS). The upwelling of mCDW also accelerates sea ice melting, creating polynyas (i.e., open areas surrounded by sea ice) that harbour productive and long-lasting phytoplankton blooms in spring-summer – the Amundsen Sea Polynya (ASP) shows the highest annual net primary production rate per unit area among Antarctica polynyas. The WS is part of the wind-driven Weddell Gyre, which is a crucial component of the global oceanic circulation as a primary formation region of deep-water masses. In the southern and western parts of the WS, dense, saline, cold shelf water forms through sea ice formation and brine rejection in winter, which subsequently descends along the continental slope, eventually exiting the WS through the Scotia Sea, contributing to AABW and the global ocean conveyor belt. Both the AS and the WS hold the potential to influence or be influenced by the global biogeochemical cycles of trace metals. Given the lack of dissolved and particulate data of Zn and Cd in the AS, the impacts of mCDW intrusion and ice shelf melting on the cycling of these two elements and their biogeochemistry in the ASP are understudied. In Chapter 2, I report the first combined dataset of dissolved and particulate Zn and Cd in the AS and evaluate potential external sources. The study reveals that upwelled mCDW is the primary source of dissolved Zn (dZn) and Cd (dCd), with continental shelf sediments being a modest source. Ice shelf melting from the DIS and sea ice melting are not significant contributors for dZn and dCd. Additionally, I compare the relative contributions of particulate Zn and Cd using two common approaches for evaluating particle composition: the operationally-defined labile and refractory fraction, and the elemental composition-based biogenic and lithogenic fraction. Findings indicate that in productive bloom regions like the ASP, the labile fraction does not always equate to the biogenic fraction. This discrepancy is attributed to inaccuracies in representing true phytoplankton uptake ratios through the slope of dissolved metal-phosphate relationships, and varying phytoplankton communities. Lastly, this study suggests that local variations in Fe availability can influence the uptake ratios of Cd and Zn across short spatial scales in the ASP, with different phytoplankton groups (such as haptophytes and diatoms) having distinct Zn, Cd, and P quotas. These insights into marine cycling and phytoplankton uptake ratios of Zn and Cd in the AS contribute to a deeper understanding of their biogeochemistry in Antarctic phytoplankton blooms.Although a handful of studies of dissolved Fe concentration ([dFe]) in the open Southern Ocean have contributed to our understanding of the biogeochemistry and marine cycling of Fe, the identification of external sources of Fe, notably sedimentary input, and the biogeochemical processes that affect Fe cycling in Antarctic waters remain relatively unclear. In Chapter 3, I employ a combination of [dFe] and Fe isotopic composition (δ56Fe) to identify Fe sources and understand the biogeochemical processes in the region, especially around the DIS and within the ASP phytoplankton blooms. Firstly, I characterize two distinct sedimentary mechanisms – reductive dissolution (RD) and non-reductive dissolution (NRD). Along with upwelled mCDW, these two sedimentary sources are the primary Fe input in the AS, potentially fuelling phytoplankton blooms in the ASP. Secondly, I explore how rapid melting of the DIS impacts Fe cycling, hypothesising enhanced preservation of lithogenic colloidal Fe(III) from shelf melting and sedimentary NRD, and the differential loss of Fe2+ in combination with Fe-binding ligands. These factors predominantly influence [dFe] and δ56Fe beneath the ice shelf system. Lastly, I observe distinct Fe isotopic fractionation within two phytoplankton blooms, dominated by haptophytes and diatoms. This indicates differences in uptake mechanisms and pathways between species and suggests that biogeochemical processes like ligand complexation, scavenging, and remineralization play crucial roles in the highly productive ASP. Overall, Chapter 3 offers a further understanding of external Fe sources and biogeochemical processes in the AS. It provides a baseline for how changing conditions in Antarctica might affect Fe cycling in the Southern Ocean and beyond. Building on the insights from Chapter 2, in Chapter 4 I delve deeper into the sources of dZn and dCd in the AS by examining the isotopic composition of Zn (δ66Zn) and Cd (δ114Cd). This analysis aligns with the findings from Chapter 2, which utilized dissolved concentrations of Zn ([dZn]) and Cd ([dCd]). These findings reconfirm that the primary source of dZn and dCd in the AS is upwelled mCDW, with minimal contributions from continental sediments and ice shelf meltwater. In the WS, I also explore the changes in [dZn], [dCd], δ66Zn, and δ114Cd during the formation of dense water, particularly focusing on the precursor of Weddell Sea Bottom Water (pre-WSBW), Warm Deep Water (WDW), and Weddell Sea Deep Water (WSDW). The findings indicate that physical mixing of these water masses is the key factor influencing the evolution of [dZn] and [dCd] in the process of WSBW formation. Interestingly, this mixing process results in almost no change in δ66Zn and δ114Cd, as these water masses share similar isotopic compositions. Lastly, the chapter sheds light on the variability in isotopic signatures in the surface waters of the AS and WS. This variability underscores the differences in surface processes affecting the cycling of Zn and Cd in Antarctic waters, such as adsorption and ligand complexation for Zn, and biological uptake for Cd. Besides Zn and Cd, I also focus on Fe in the WS, notably its potential sources and the processes influencing [dFe] and δ56Fe during the formation of WSBW in Chapter 5. I find that meltwater from the Antarctic Peninsula and continental shelf sediments contribute substantially isotopically light Fe to the shelf water, predominantly through RD, accounting for up to 90% of the input. This primary isotopically light signature in shelf water is initially preserved across varying [dFe]. However, as the dense shelf water moves down the continental shelf slope, this signature gradually shifts towards heavier signatures. I suggest that this transition in δ56Fe during the transformation of shelf water into pre-WSBW is mainly driven by physical mixing with WDW and WSDW. This implies that the sediment?and glacier-derived light Fe signature is likely diluted in the northward-flowing WSBW and AABW, underscoring the role of physical mixing in Fe cycling in these bottom waters. Additionally, I observe a consistent δ56Fe minimum at intermediate depths in the WS, indicative of long-range transport of an isotopically light Fe signal. Previous hypotheses attributed this to either subsurface remineralization or sedimentary light input, where this research supports the latter. My work suggests that sedimentary derived light Fe from the Antarctic Peninsula shelves can be preserved and transported over long distances, offering a distinct endmember for tracing sediment-derived δ56Fe beyond the shelf region. This insight enhances the potential of using δ56Fe as a tracer for sediment supply in the open ocean.Overall, this thesis presents the first set of observational data for δ56Fe in both the AS and the WS, as well as the first data for δ66Zn and δ114Cd in the AS and greatly improved coverage of the WS. These insights are pivotal in understanding the external sources influencing the distributions of these elements in the Southern Ocean. Furthermore, this thesis delves into the isotopic systematics of these elements in the ocean. It illuminates the fractionation effects caused by various biogeochemical processes, such as biological uptake, adsorption, and ligand complexation. This investigation not only enhances our understanding of these processes but also provides valuable perspectives on using isotopic compositions as tools to investigate biogeochemical processes. Finally, given the limited availability of observational trace element data in the Southern Ocean and coastal Antarctica, the findings and discussions presented in this thesis can be applied to broader biogeochemical studies and enhancing the use and projection capability of environmental modelling |
|
