Salinity and redox alternations in the northwestern Baltic proper during the late Holocene

Abstract

Selected geochemical parameters and siliceous microfossil assemblages in Baltic Sea sediments are presented which reflect past variations in redox conditions, salinity and primary production. The sediments were deposited during the freshwater Ancylus Lake (9500‐8000 14C BP) and brackish Litorina Sea (8000‐3000 BP) stages of the Baltic. The diatom record shows that surface‐water salinity increased further at c. 7000‐6500 BP, although smaller amounts of brackish water entered the basin from c. 8000 BP onwards. Attempts to use exchangeable Mg as a palaeosalinity indictor were not applicable. Gross primary productivity increased along with salinity, which has been interpreted as an effect of nutrient enrichment in the photic zone. This led between c. 6500 and 4500 BP to a high accumulation of organic carbon, anoxic or nearly anoxic bottom conditions and formation of laminated deposits. Certain laminae consist of alternating layers of organic and minerogenic material and were probably formed annually, i.e. in the manner of varves. The laminated successions are distinguished by enrichments of V, Cu and especially Mo. The highest Mo content occurs in the core from the greatest water depth, an effect of anoxic conditions during deposition. The Fe/Mn ratio was shown to be ambiguous as an indicator of past redox conditions. Since biogenic silica shows large variablity in contemporaneously deposited sediments, this parameter cannot be used as a proxy for the past production of siliceous algae in the Baltic Sea.

Sediment Bacterial Communities Reflect the History of a Sea Basin

“…The high preservation potential of the Litorina Sea sediment was due to an inflow of saline water through the Danish Straits, resulting in a halocline formation [6]. Consequently, oxygen deficiency slowed down the mineralization of organic matter [7]. The Litorina transgression maximum, occurred in the eastern GOF between app.…”

“…The halocline resulted in oxygen depletion in the bottom sediments, slowing down the mineralization of the organic matter deposited [7]. The transgression and salinity maxima occurred in distinct areas of the Baltic Sea at different times; e.g.…”

Iron isotope variations in Holocene sediments of the Gotland Deep, Baltic Sea

“…Additionally, $5% jarosite was detected by Mö ssbauer spectroscopy in the sample from 175 cm depth, which provides evidence of oxidation, probably due to storage and drying of the sample. The amorphous or poorly crystalline material likely consists of biogenic silica, which is present in sediments of the Baltic Proper at concentrations of 1-7% (Sohlenius and Westman, 1998), of organic C (up to 10%; Sohlenius et al, 2001), and of amorphous Fe-oxyhydroxides. Besides, FeS (mackinawite) is probably present at low concentrations throughout the core (Boesen and Postma, 1988;Sohlenius et al, 2001), whereas the highest FeS contents of $1.8% were determined in the uppermost part of the Ancylus Lake unit (Boesen and Postma, 1988).…”

Temporal variability and environmental availability of inorganic constituents in an Antarctic marine sediment core from a polynya area in the Ross Sea

“…Figure 2 reports the depth profiles for the Fe/Mn, Cu/Zn and Al/Ti ratios, and Ba excess . The first ratio has been used by various authors (Tolonen and Merila¨inen 1983;Sohlenius and Westman 1998) as a paleoredox indicator in sediment cores. Since manganese oxides are dissolved from the sediment at a higher redox potential than iron oxides, a high Fe/Mn ratio may be the result of reducing conditions at the sediment-water interface.…”

“…Nevertheless the use of this ratio is limited, as manganese and iron can form minerals under both anoxic and oxic conditions, depending also on the bacterial reduction of sulphates to sulphides. Also the Cu/Zn ratio can be used to describe sediment redox conditions (Sohlenius and Westman 1998); its application is restricted to areas where an anthropogenic input of these elements can reasonably be excluded, like the area considered in this study. A clear difference of both ratios is evident between the uppermost (from the top to À7 cm) and the deeper parts of the core.…”