Anatomy of the Major Baltic Inflow in 2014: Impact of manganese and iron shuttling on phosphorus and trace metals in the Gotland Basin, Baltic Sea. (1st July 2021)

• Record Type: Journal Article
• Authors: Dellwig, Olaf; Wegwerth, Antje; Arz, Helge W.
• Is Part Of: Continental shelf research. Volume 223(2021)
• Publication Date: 2021-07-01
• Subjects: Major Baltic inflow -- Phosphorus -- Redox-sensitive trace metals -- Hypoxia -- Gotland Basin -- Baltic sea; Continental shelf -- Periodicals; Submarine geology -- Periodicals; 551.41
• DOI: 10.1016/j.csr.2021.104449
• Languages: English
• ISSNs: 0278-4343

Abstract

Abstract: The deep basins of the Baltic Sea experienced dramatic climate- and eutrophication-induced redox changes between oxic and euxinic water column conditions. Irregularly appearing Major Baltic Inflows (MBI) carrying North Sea waters to the Baltic Sea are partly able to oxygenate euxinic bottom waters within several weeks. After an absence of ~10 years, an exceptional strong inflow event in December 2014 reached the Gotland Basin in early March 2015 providing the opportunity of detailed studies on the impact of fast redox shifts on elemental cycles. Here we report on P and trace metal (Co, Mo, U, V and W) dynamics in the water column, pore water, and sediment of the central Gotland Basin over a 4.5 year-long observation period. These time series record the entire redox development including the primary MBI-related oxygenation, the transitional development to anoxia before a second inflow pulse, and the final re-establishment of euxinia. Samples obtained in August 2012 represent the previous euxinic stagnation period not affected by MBI's since nearly a decade. Concentration measurements and a budget calculation suggest only a limited impact of the MBI on the water column P inventory and the corresponding eutrophication level of the Gotland Basin. This is due to insufficient Fe availability preventing efficient trapping of phosphate during water column oxygenation. In addition, the fast recovery of reducing conditions allowed only a short-term storage of deposited Mn–Fe–P mineral phases in the surface sediment. Due to pronounced scavenging by Mn oxides, substantial water column depletion occurred intermittently for V and W, but again, re-establishing reducing conditions favored Mn oxide dissolution and the release especially of W back into the water column. Mo and U were less affected by Mn and Fe shuttling and Mo even showed an increasing water column inventory due to replenishment by intruding North Sea waters. In contrast, the water column inventory of Co remained on an approx. 50% level about 4.5 years after the inflow suggesting a significant sedimentary burial of Co within the course of the inflow event. Although sedimentary Mn carbonate layers, which were commonly formed in concert with the frequent inflows between ~1960 and 1980, were missing or only weakly developed after the single inflow events in 1994, 2003, and 2014, dated short cores from the study area reveal clear Co and less pronounced V enrichments during the latter inflows. Because Mo, U and W are synchronously depleted compared with their normal sapropel levels, the combination of these trace metals and Co may help to identify past MBIs not reflected by Mn carbonate presence. Highlights: Inflow-related dynamics of P and trace metals were studied in the Gotland Basin. Limited impact of water column oxygenation on P inventory during a 4.5 year period. Intruding North Sea waters replenished Mo and U water column inventories. A substantial fraction of Co and V was retained in the sediments after the inflow. Different sediment signatures of Co, Mo, U, and W serve as proxy for past inflows.