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Title: Copper biogeochemical cycle and the organic complexation of dissolved copper in the North Atlantic
Author: Gourain, Arthur
ISNI:       0000 0004 9352 0821
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2020
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The copper biogeochemical cycle has been investigated in the open ocean with a focus on the North Atlantic. Two GEOTRACES sections have been used in order to study the distribution of dissolved copper (DCu) and its organic speciation in the North Atlantic. The GEOTRACES GA01 section was used as a natural laboratory in order to reveal the processes shaping the DCu distribution in the open ocean. Indeed, two contrasting regions were crossed: the east part of the section with increasing DCu concentrations with depth and the Arctic basins (Labrador and Irminger) in the west with homogeneous DCu concentrations with depth. The variability of distribution was driven by the organic speciation, elevated ligand concentrations in the east basin were stabilising copper in the dissolved form, allowing it to accumulate at depth. Moreover, elevated ligands concentrations were associated with an enrichment of copper compared to phosphorus in particles (> 0.45 μm). This enrichment was triggered by an increasing contribution of authigenic particulate copper over the particulate copper (PCu) cycle. Authigenic PCu was predominant at depth of every basin but the highest contributions were observed in the east Atlantic basin. Elevated authigenic PCu at depth demonstrated an intense reversible scavenging thus leading to elevated DCu concentrations. This shows for the first time the impact of reversible scavenging on natural copper samples. The GEOTRACES GA13 section occurred over the Mid-Atlantic Ridge to study the impact of hydrothermalism on trace metal biogeochemical cycle. Along the cruise, multiple known hydrothermal sites were sampled with a focus on the deep ocean. Within the hydrothermal plumes identified using dissolved manganese (DMn), no variation in the DCu distribution and organic speciation was observable. The absence of DCu signal within the plumes resulted from a combination of dilution with surrounding water masses and removal by coprecipitation with sulphide. The lack of signal of copper complexation is more complex due to the combination of dilution, production of ligands and degradation of organic matter in the close vicinity of the vents. An inter-comparison between two GA13 stations and two GEOTRACES section (GA01 and GA03) showed the comparability of DCu and ligand profiles. The small variability observed between profiles was explained by local and seasonal processes affecting the (sub-)surface layer. Copper profiles from a global dataset have been studied in order to constrain the DCu distribution in all the world's ocean. The dataset combines previously published datasets with the GEOTRACES IDP 2017 and covers all oceans basins. The distribution of DCu is linear with depth in most of the ocean with a Spearman's rank correlation coefficient higher than 0.8 for 72% of the stations. But the slope of profiles varies between basins; the Atlantic Ocean being characterised by low slope while the Pacific Ocean shows elevated slopes. The variability of slope is driven by the deep DCu concentrations, elevated DCu concentrations in the deep Pacific are leading to an increase of the slope. A diagnostic framework was applied to study the components affecting the copper distribution: the regenerated, preformed and reversible scavenged pools. The regenerated copper pool has the same behaviour as the macronutrients, like phosphate; regenerated copper represents less than 100 % of the total DCu pool. Analysis of the scavenged component demonstrates the transition of scavenging from a sink of copper in surface waters to a source of copper at depth. The switch between a net sink to a net source of DCu occurs at around 2250 m depth in the open ocean. Reversible scavenging is an important source of DCu at depth, shaping its unique linear distribution with depth.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral