Phosphorus forms and reactive iron in lateglacial, postglacial and brackish-water sediments of the Archipelago Sea, northern Baltic Sea

Glacial transport is an important agent of P and reactive Fe delivery to the ocean margins. However, whereas P may travel through salinity gradients, reactive Fe is substantially trapped in estuaries. Studies of glaciolacustrine to brackish-water depositional successions elucidate processes associated with the glacial P and reactive Fe flux into the oceans. The sedimentary P species, ‘reactive’ Fe-oxide-associated Fe (and Al and Mn), C, N and S geochemistry for sediment cores from three sites in the Archipelago Sea (northern Baltic Sea) were determined in order to understand the distribution and coupling of these geochemical components in the glaciolacustrine to post-glacial lacustrine to brackish-water depositional succession. The up to 6 m long cores record deposition after 11 350 cal. BP. Previous studies suggest that the studied depositional succession records development common for the deglaciation phase of large, low relief, epicontinental basins.All the studied geochemical components are strongly coupled in the basal glaciolacustrine rhythmites, which reflect their rapid deposition by glacial meltwaters. Low Corg/Preac ratios suggest that a part of the rhythmite material is reworked from pre-glacial deposits. Delivery rates are drastically lower in the successive post-glacial lacustrine setting due to the decrease of sedimentation rate by an order of magnitude. Decoupling of reactive Fe and associated components in the post-glacial lacustrine clays reflect increasing contribution of sediment from the emerging shoreline and rivers, but also post-depositional overprinting by sulphidization. The onset of brackish-water conditions soon after 7600 cal. BP led to 1) decoupling of reactive Fe from the detrital flux due to the establishment of salinity gradients at the river mouths, and 2) increased nutrient availability and production in water, as the intensified accumulation and burial of reactive P species, organic C and N indicate. The higher organic deposition led to deteriorated seafloor oxygen conditions; alternating thinly-laminated to strongly-bioturbated lithofacies characterise the brackish-water muds, and record temporal changes in the seafloor oxygenation. The temporal changes in the seafloor oxygen conditions appear to be broadly linked to the Holocene climate variability; brackish-water muds at the northern end of Paimionlahti Bay record anoxia during the Medieval Warm Period (1250–450 cal. BP), oxic conditions during the ‘Little Ice Age’ (450–300 cal. BP), and anoxia until the present day. On a finer scale, the redox histories in adjacent sub-basins differ considerably due to patchy topography, sluggish bottom water exchange and high organic deposition in the area. Terrestrial organic influx can temporally overprint marine organic deposition in areas close to river mouths.