Ultra-high resolution sediment sequence from coastal Littorina sea spanning the Holocene thermal maximum – environmental magnetic study from Kurikka, Southern Ostrobothnia

Both natural and anthropogenic factors cause present day hypoxia in the Baltic Sea basin (BSB), leading to the formation of sulfide sediments (Zillén et al. 2008). Moreover, acid sulfate soil is a common environmental challenge in areas of raised littoral deposits. Before present and medieval times, the BSB has experienced widespread hypoxia during the Holocene thermal maximum (HTM, ca. 8–4 ka) (Zillén et al. 2008). The extensive laminated sulfate sediments have thought to represent hypoxic conditions in the Bothnian bay throughout the early Littorina sea (Sohlenius & Öborn 2004). Yet past occurrences of BSB hypoxia have been recorded primarily in deep basin sediments cores with resolutions seldom exceeding ~0.01 cm/a. Little is still known of the seafloor redox condition variability in the littoral zone during HTM (van Helmond et al. 2017).

A promising novel proxy for oxygen depletion and redox zonation is occurrence of greigite (Fe3S4), a magnetic mineral that forms in sulfidic and methanogenic redox zones of the sediment-water interface (Roberts et al. 2018). Greigite has been identified from BSB sediments, (e.g. Sohlenius 1996, Kortekaas 2007, Lougheed et al. 2012, Reinholdsson et al. 2013) but the formation pathways are still under discussion.

Here, the environmental magnetic (EnvMag) methods are used to study the occurrence and characteristics of greigite in a HTM ultra-high resolution (0.75 cm/a) sediment succession from Kurikka, S. Ostrobothnia, Finland. A 40-m long sediment core was analyzed for lithology, magnetic susceptibility, organic matter content, grain size and EnvMag characteristics to construct the stratigraphy and magnetic mineral composition. The sediment core covers local deglaciation, Ancylus lake and Littorina sea evolution until isolation from the sea (~10 800–4500 a).

Authigenic single-domain greigite was found from Littorina sediments with signs of sulphidic redox origin. The results propose diagenetic origin for greigite in BSB littoral sediments, contradicting the earlier suggestion – derived from deep sea environment - of bacterial-only origin for greigite in the BSB (Reinholdsson et al. 2013). Furthermore, unlike previous studies, no greigite was found from Ancylus clays. The findings indicate different greigite formation processes for the littoral zones compared to those of deep-sea basins in the BSB. This suggests that even within the same basin and sediment units, the formation pathways of greigite differ.

Susceptibility was coupled to EnvMag results and used to assess hypoxia variation along the sediment sequence. Hypoxia was found to be frequent although discontinuous during the HTM in the shallowing Littorina sea bay. The largescale trends of hypoxia were perennial, but possibility for (sub)annual variation could not be ruled out. Two periods of centennial hypoxia were identified, while the varying greigite concentrations indicate unstable oxygen conditions. Signs of short-termed oxic conditions between low-oxygen periods (~6500–7000 cal BP) were identified. The period has been previously identified with higher salinity levels and relative sea level rise in the BSB.