『Abstract
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 11350 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.
Keywords: phosphorus; reactive iron; lacustrine; brackish-water;
glacial sediments; anoxic sediments; Archipelago Sea; Baltic Sea』
1. Introduction
2. Regional setting
3. Materials and methods
3.1. Fieldwork
3.2. Laboratory analysis
3.3. Sediment chronology and accumulation rates
3.4. Statistical analysis
4. Results and interpretation
4.1. Core lithology and sediment chronology
4.2. Geochemistry
4.2.1. Glaciolacustrine rhythmites (Dragsfjard(後のaの頭に¨)
Alloformation)
4.2.2. Post-glacial lacustrine clays (Sandon(oの頭に¨)
Allomember)
4.2.3. Brackish-water organic-rich muds (Nauvo Alloformation)
5. Discussion
5.1. Waning glacial influence on deposition
5.2. Onset of brackish-water conditions
5.3. Direct climatic influence on deposition
6. Conclusions
Acknowledgements
Appendix A
Appendix B. Supplementary data
References