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doi:10.2204/iodp.pr.347.2014

Preliminary scientific assessment

Paleoenvironmental evolution of the Baltic Sea Basin through the last glacial cycle

The overarching objective of Expedition 347 was to gain a deeper understanding of the paleoenvironmental evolution of the BSB through the last glacial cycle. The expedition set out to recover sediment sequences covering the time from MIS 5 to the present at as high resolution as possible. In order to meet this aim, nine sites were drilled during the expedition, selected on the basis of thorough seismic presite surveys and with a geographic distribution aimed to register the in- and outflows from the BSB, as well as the sediments deposited within the basin itself.

After the expedition, the following OSP carried out visual inspection and description of the >1600 m of core retrieved, measurements of physical properties, and intense sampling of the cores. Initial biostratigraphic and lithologic analyses provided a preliminary chronology that, together with the visual appearance of the sediment, was used as a guide for further sampling.

The original proposal for Expedition 347 stated four major research themes, listed below. To explore these four themes, further laboratory work will be carried out during the coming year.

In general, the expedition was very successful and many of the research questions formulated in the original proposal will be explored and the objectives met. In the following, we give a brief overview of the preliminary results.

Climate and sea level dynamics of MIS 5, including onsets and terminations

The lowermost part of the cored sequences from Sites M0059 and M0060 revealed biostratigraphic indications of possible MIS 5 age, and sufficient material was collected to perform OSL dating. The stratigraphic complexity of the two sites calls for intense laboratory investigations and dating, both by the OSL method and by pollen stratigraphy, before more conclusions can be drawn regarding dating. At this point, however, we conclude that the prospect of a deeper understanding of MIS 5 will be provided by the cored material.

Complexities of the last glacial (MIS 4–MIS 2)

The sediments recovered between ~80 and 120 mbsf at Site M0059 and between ~90 and 150 mbsf at Site M0060 display intriguingly complex lithologies that, together with the preliminary biostratigraphic and geochemical data, hold important environmental information about the last glacial stages, MIS 4–MIS 2. Sediments were recovered at Sites M0064, M0065, and M0066 that are older than late glacial and will provide very specific information, especially on conditions within the BSB during parts of MIS 4–MIS 2.

Deglacial and Holocene (MIS 2–MIS 1) climate forcing

The glacial varve sequences recovered at Sites M0063, M0064, and M0065 will greatly improve knowledge of the deglaciation of the southern and central BSB and will provide new general understanding of the behavior of the ice sheet during MIS 2. The varved sequences from Sites M0059 and M0060 may also contribute to this knowledge, although they can probably not be directly correlated to the existing Swedish glacial varve chronology and thus not used for any age determination. The cored varved sequences from Sites M0061 and M0062 will give new insights into the late Holocene history of the northern part of the BSB.

An unexpected long Holocene sequence from Site M0059, together with an extremely expanded Holocene sequence from Site M0063, will allow reconstructions of, for example, climate response and other external forcing mechanisms with a resolution that was previously not possible. The prospect of reconstructing Holocene environmental variations with such high resolution as provided by these sediment archives will give unique new knowledge on how the anthropogenically unaffected BSB ecosystem responded.

Deep biosphere responses to glacial–interglacial cycles

Four of the drilled sites were selected for the study of the deep biosphere and how the buried microbial communities responded to major shifts in the Baltic Sea environment in the past and to varying lithologies and geochemical stratification in the present. Separate holes were drilled at Sites M0059, M0060, M0063, and M0065 to sample for microbiology, interstitial water chemistry, and ephemeral geochemical parameters such as organic biomarkers.

For the first time during IODP, two different methods were used offshore for cell counts, both using fluorescent staining of DNA. The IODP standard AODC method was compared to an FCM method that relied on a quantitative extraction of cells from the sediment. It was an important result that the two methods gave very similar results in the majority of sediments. Microbial cell counts are generally extremely high, particularly in the organic-rich Holocene deposits younger than ~8000 y that yielded the highest microbial abundances yet recorded by IODP.

A perfluorocarbon tracer was used while drilling the microbiology holes to evaluate potential contamination with cells from the drill fluid. A new application of this technique with sampling of liner fluid from each drilled piston core and a modified preparation of samples for gas chromatography improved the standard approach significantly and helped guide the selection of samples for subsequent microbiological analyses.

A large number of samples were taken offshore from the microbiology holes, using microbiological techniques. These were shipped during and after the expedition to requesting laboratories around the world and, based on the many novel ideas and techniques that will be applied, important new information on microbial life in the deep biosphere is expected.

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