Authors:	Huang, Xizhi
Title:	Tracking the history of Baltic Sea hypoxia with bivalve shells
Online publication date:	25-Apr-2024
Year of first publication:	2024
Language:	english
Abstract:	Deoxygenation is a growing issue in global open oceans and coastal waters leading to hypoxia, of which the Baltic Sea is one of the largest oxygen-deficient settings in the world. As a model region, understanding the past occurrence of Baltic Sea hypoxia is crucial for informing present- day coastal perturbations and developing future mitigation strategies. To date, reconstructing the history of hypoxia in the Baltic Sea is almost exclusively on proxies archived in sediment cores, which have low temporal resolution and poor dating control, limiting the ability to provide detail information on dissolved oxygen (DO) variability and hypoxic events. To reliably predict the DO development in the Baltic Sea, it is of paramount important to identify the leading to oxygen depletion based on an accurate understanding of historical trends in DO. Hence, developing temporally well-constrained and highly resolved DO proxy archives that can overcome the limitations of sedimentary records is imperative. The shells of bivalve mollusks may fulfill this task as they can function as precisely dated and seasonally to interannually resolved archives of environmental changes. Specifically, an redox-sensitive element such as Mn has the potential to be used as a surrogate for DO. However, it remains a challenge to quantitatively interpret the environmental signatures from elemental impurities in bivalve shells. Before shell Mn/Ca values can be used to quantify DO concentrations, this proxy needs to be calibrated. The present study was based on shell materials of long-lived bivalve specimens and adopted sclerochronological techniques (Chapter 2). The results of this project comprise four manuscripts published in international, peer-reviewed scientific journals. In Chapter 3, the ocean quahog Arctica islandica, an extremely long-lived and low- oxygen adapted bivalve species commonly used for paleoclimate reconstructions, was studied. All specimens of A. islandica were collected alive at the same time from the Mecklenburg Bight, SW Baltic Sea. The results showed that Mn/Cashell values were statistically significantly negative correlated with DO levels in the water column (r = - 0.68; R2 = 0.46, p < 0.0001). This relationship reflects that the Mn in shells is mainly derived from the dissolved Mn in the surrounding waters. These findings demonstrated the great potential of bivalve shells A. islandica as archives for tracking the history of hypoxia in the Baltic Sea. In Chapter 4, it was investigated whether shells of Astarte borealis and Astarte elliptice can record DO in a similar way as those in A. islandica and thus serve as alternative DO archives. In specimens of all three species living on the seafloor in the same region of the Fehmarn Belt in the SW Baltic Sea, the Mn/Cashell data are statistically significantly linked to DO concentrations (r = - 0.66 to – 0.75, R2 = 0.43 to 0.5, p < 0.0001). These relationships can be used to infer DO data, with A. elliptica providing a slightly higher precision (± 1.5 mL/L) of DO data than A. islandica or A. borealis (± 1.6 mL/L), suggesting that DO reconstructions can be conducted with all three species. Besides chemical analyses, Chapter 5 explored if the shell ultrastructure, specifically disturbance lines, can be used as an alternative proxy to track the frequently and severity of past deoxygenation events. The results revealed that oxygen depletion led to the formation of Mn- rich disturbance lines that become more prominent and consist of smaller and more elongated biomineral units (BMUs). Although the relationship between DO and BMU morphometry was statistically significant (p < 0.05), the explained variability (< 1.5 %) was too small, suggesting that a combination of Mn/Cashell data is required if shell ultrastructure properties are employed as a proxy for deoxygenation events. The faithfulness of DO reconstructions based on shell Mn/Ca values was corroborated by an extensive dataset across time, space and bivalve species in previous studies (Chapter 3 and 4). Therefore, in Chapter 6, long-term and high-resolution Mn/Cashell chronologies were constructed from shell A. islandica that covered the second half of the 19th century as well as the late 20th century to study if Mn/Cashell values of historical shells were significantly different from those of modern times. All specimens were live-collected at water depths of 23.5 to 25 m in the Mecklenburg Bight, SW Baltic Sea. By analyzing Mn/Cashell data in terms of intensity and variability, it is clear that seasonal deoxygenation had already occurred in the SW Baltic Sea in the mid-19th century, however, it was less frequent than in the late 20th century. In addition, with the help of other environmental proxies in the shell (i.e., Ba/Cashell values and annual increment width), the causes of oxygen depletion in the SW Baltic Sea were inferred. Specifically, strong anthropogenic nutrient input was probably the main cause of oxygen depletion shortly after the onset of the Industrial Revolution (ca. 1850), whereas the present DO level at bottom waters seems to be the result of a combined effect of eutrophication and Major Baltic Inflows (MBIs). In conclusion, the works of this thesis calibrate and develop shell Mn/Ca ratio as a reliable DO indicator and apply this technique to infer the hypoxia record in the SW Baltic Sea over the past centuries with unprecedented temporal resolution. Future research should broaden the spatial and temporal scales for retrospective monitoring of the Baltic deoxygenation.
DDC:	333.7 Natürliche Ressourcen 333.7 Natural resources 500 Naturwissenschaften 500 Natural sciences and mathematics 550 Geowissenschaften 550 Earth sciences 560 Paläontologie 560 Paleontology
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-10274
URN:	urn:nbn:de:hebis:77-openscience-a3155124-028a-45f3-b22a-80056b95dd899
Version:	Original work
Publication type:	Dissertation
License:	CC BY
Information on rights of use:	https://creativecommons.org/licenses/by/4.0/
Extent:	xxvi, 200 Seiten ; Illustrationen, Diagramme
Appears in collections:	JGU-Publikationen