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A study of the seasonal and interannual variability of phytoplankton and zooplankton assemblages in a significant marine ecosystem

Kämpf Jochen, Newman Michelle, Doubell Mark, Möller Luciana, Baring Ryan, Shute Alex, Rodriguez Ana

Oceanologia

2023

Vol. 65 (2)

434--451

The eastern Great Australian Bight (GAB) is a significant marine ecosystem, featuring a range of marine mammals and large pelagic fish including blue whales, sharks and tuna. Previous research has classified the region as generally oligotrophic, apart from late austral summer months when seasonal upwelling triggers phytoplankton blooms in the region. Based on multi-year field observations, this study analysed the interannual and interdecadal variability of the plankton community structure in this region. Pigment data indicate that nano- and pico-phytoplankton generally dominated the phytoplankton community structure with averages of 39% and 30% of the total biomass, including a relatively large proportion of nanophytoplankton (cryptophytes, haptophytes and prasinophytes) with cell sizes <5 µm, not resolved in microscopic cell counts. Nano- and pico-phytoplankton alone contributed ∼0.3 mg/m3 to the chlorophyll-a signal and therefore sustained an overall mesotrophic environment year-round. Distinct diatom blooms developed during the upwelling season within concentrated subsurface layers where chlorophyll-a concentrations increased to >1 mg/m3, characterising eutrophic conditions. The biomass of diatoms increased from <10% to ∼30% of total biomass. Diatom blooms coincided with relatively high abundances of three dominant zooplankton species (Oithona similis, Penilia avirostris and Microsetella norvegica) and/or the dinophyta Noctiluca scintillans, but events of high zooplankton abundance also occurred outside the upwelling season. The observational findings also show the occurrence of significant subsurface phytoplankton blooms in late spring, not reported before, that may also contribute to the ecosystem functioning of the region.

Origin and significance of early-diagenetic calcite concretions and barite from Silurian black shales in the East European Craton, Poland

Bojanowski Maciej J., Kędzior Artur, Porębski Szczepan J., Radzikowska Magdalena

Acta Geologica Polonica

2019

Vol. 69, no. 3

403--430

The Silurian Pelplin Formation is a part of a thick, mud-prone distal fill of the Caledonian foredeep, which stretches along the western margin of the East European Craton. The Pelplin Formation consists of organic carbon-rich mudstones that have recently been the target of intensive investigations, as they represent a potential source of shale gas. The Pelplin mudstones host numerous calcite concretions containing authigenic pyrite and barite. Mineralogical and petrographic examination (XRD, optical microscopy, cathodoluminoscopy, SEM-EDS) and stable isotope analyses (δ13Corg, δ13C and δ18O of carbonates, δ34S and δ18O of barite) were carried out in order to understand the diagenetic conditions that led to precipitation of this carbonate-sulfide-sulfate paragenesis and to see if the concretions can enhance the understanding of sedimentary settings in the Baltic and Lublin basins during the Silurian. Barite formed during early diagenesis before and during the concretionary growth due to a deceleration of sedimentation during increased primary productivity. The main stages of concretionary growth took place in yet uncompacted sediments shortly after their deposition in the sulfate reduction zone. This precompactional cementation led to preferential preservation of original sedimentary structures, faunal assemblages and earlydiagenetic barite, which have been mostly lost in the surrounding mudstones during burial. These components allowed for the reconstruction of important paleoenvironmental conditions in the Baltic and Lublin basins, such as depth, proximity to the detrital orogenic source and marine primary productivity. Investigation of the concretions also enabled estimation of the magnitude of mechanical compaction of the mudstones and calculation of original sedimentation rates. Moreover, it showed that biogenic methane was produced at an earlydiagenetic stage, whereas thermogenic hydrocarbons migrated through the Pelplin Formation during deep burial.