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New geophysical, oceanographic data and studies on sediment core material strengthen the hypothesis that winter-time deep-water formation / convection played a critical role in seabed dynamics and bottom water ventilation in the Baltic Sea during the last 7000 years. Moreover, we suggest that this process was also critical for marked environmental changes observed and reconstructed for the last 150 years, the transition from the cold Little Ice Age towards the Modern Warm Period.Our inferences are based on a comparison of instrumental data comprising oceanographic measurements (temperature, salinity, oxygen), air temperature data from Stockholm and sediment proxy records. A solid proxy for bottom water salinity changes are benthic foraminiferal counting data. X-ray fluorescence (XRF) scanning data of short sediment cores together with organic carbon content data are used to evaluate the input of terrigenous particles at the respective sites. XRF data are also used to identify manganese-carbonate layers that form at the sediment surface when anoxic bottom waters are ventilated. A sound chronostratigraphy for the last c. 150 years critical for a comparison between proxy and instrumental data is achieved by combining radionuclide, inorganic and organic pollutant downcore data.Our proxy records and the instrumental data indicate that during climate warming since AD 1850 the winter-time deep-water formation / convection decreased steadily as evident from deep basin study sites (e.g., in the Landsort Deep). A marked environmental change occurred in the late 1950s when the input of re-worked terrigenous material stopped suddenly and the bottom water conditions switched from oxic to hypoxic at different water depths at all sites (deeper than 160 m) in the Baltic Proper. This switch occurred shortly after the strong inflow of saline waters in the early AD 1950s when highest salinities were observed (instrumental data) and reconstructed (benthic foraminifera) in all sub-basins of the Baltic Proper. We argue that stratification strengthened markedly resulting from this pronounced increase in bottom water salinity, and the already rather weak winter-time deep-water formation / convection collapsed. The collapse led to a stop of fine-grained material input and bottom water ventilation decreased.During the late 1980s, however, bottom water salinities decreased and therefore stratification weakened cause of a lack of significant inflows. This together with colder winter air temperatures lead to a bottom water ventilation of sub-basins located north of a marked topographic seabed feature, the Baltic Sea Klint, due to the onset of the postulated winter-time deep-water formation / ventilation process.After the early 1990s only the major saline water inflow of 2003 was able to effectively ventilate the bottom waters of the sub-basins north of the Baltic Sea Klint, which is evident from instrumental and foraminiferal proxy data.There is an urgent need to include the winter-time deep-water formation process in ecosystem modelling approaches as this may help to improve the relatively poor model performance for ventilation changes in sub-basins located North of the Baltic Sea Klint.

The short sediment core EMB201/7-4 retrieved from the East Gotland Basin, central Baltic Sea, is explored here as a candidate to host the stratigraphical basis for the Anthropocene series and its equivalent Anthropocene epoch, still to be formalized in the Geological Time Scale. The core has been accurately dated back to 1840 CE using a well-established event stratigraphy approach. A pronounced and significant change occurs at 26.5 cm (dated 1956 +/- 4 CE) for a range of geochemical markers including Pu239+240, Am-241, fly-ash particles, DDT (organochlorine insecticide), total organic carbon, and bulk organic carbon stable isotopes. This stratigraphic level, which corresponds to a change in both lithology and sediment colour related to early anthropogenic-triggered eutrophication of the central Baltic Sea, is proposed as a Global Boundary Stratotype Section and Point for the Anthropocene series., The Anthropocene Review, 10 (1), ISSN:2053-0196, ISSN:2053-020X

PREMISE Understanding the adaptive capacities of species over long timescales lies in examining the revived recent and millennia old resting spores buried in sediments. We show for the first time the revival, viability and germination rate of resting spores of the diatom Chaetoceros deposited in sub-seafloor sediments from three ages (recent: 0-80 years; ancient: ~1250 (Medieval Climate Anomaly) and ~6600 (Holocene Thermal Maximum) calendar year before present. METHODS Recent and ancient Chaetoceros spores were revived to examine their viability and germination rate. Light and scanning electron microscopy and Sanger sequencing was done to identify the species. KEY RESULTS We show that ~6600 cal. year BP old Chaetoceros resting spores are still viable and the vegetative reproduction in recent and ancient resting spores vary. The time taken to germinate is three hours to 2-3 days in both recent and ancient spores, but the germination rate of the spores decreased with increasing age. The germination rate of the recent spores was ~41% while that of the ancient spores were ~31% and ~12% for the ~1250 and ~6600 cal. year BP old resting spores. Based on the morphology of the germinated vegetative cells we identified the species as Chaetoceros muelleri var. subsalsum. Sanger sequences of nuclear and chloroplast markers identified the species as Chaetoceros muelleri. CONCLUSIONS We identify a unique model system, Chaetoceros muelleri var. subsalsum and show that recent and ancient resting spores of the species buried in sediments in the Baltic Sea can be revived and used for long-term evolutionary studies. This article is protected by copyright. All rights reserved.

Climate changes over the past two millennia in the central part of the Gulf of St. Lawrence are documented in this paper with the aim of determining and understanding the natural climate variability and the impact of anthropogenic forcing at a regional scale. The palynological content (dinocysts, pollen, and spores) of the composite marine sediment core MSM46-03 collected in the Laurentian Channel was used to reconstruct oceanographic and climatic changes with a multidecadal temporal resolution. Sea-surface conditions, including summer salinity and temperature, sea-ice cover, and primary productivity, were reconstructed from dinocyst assemblages. Results revealed a remarkable cooling trend of about 4°C after 1230 cal yr BP (720 CE) and a culmination with a cold pulse dated to 170–40 cal yr BP (1780–1910 CE), which likely corresponds to the regional signal of the Little Ice Age. This cold interval was followed by a rapid warming of about 3°C. In the pollen assemblages, the decrease ofPinusabundance over the past 1700 yr suggests changes in wind regimes, likely resulting from increased southerly incursions of cold and dry Arctic air masses into southeastern Canada.

Knowledge about the timing, amplitude and spatial gradients of Holocene environmental variability in the circum-Baltic region is key to understanding its responses to ongoing climate change. Based on a multi-dating and proxy approach, we reconstruct changes in productivity using total organic carbon (TOC) contents in sediments of Lake Kälksjön (KKJ) from west–central Sweden spanning the last 9612 (+255/-114) years. An exception is the period from 1878 CE until today, in which sedimentation was dominated by anthropogenic lake level lowering and land use. In-lake productivity was higher during periods of warmer winters with shortened ice cover and prolonged growing seasons. A multi-millennial increase in productivity throughout the last ∼ 9600 years is associated with progressively warmer winters in northwestern Europe, likely triggered by the coinciding increase in Northern Hemisphere winter insolation. Decadal to centennial periods of higher productivity in KKJ tend to correspond to warmer winters during a more positive North Atlantic Oscillation (NAO) polarity, as reconstructed for the last 8000 years. In consequence, we assume our decadal to centennial productivity record from KKJ sediments for the complete ∼ 9600 years to provide a qualitative record of NAO polarity. A shift towards higher productivity variability at ∼ 5450 cal a BP is hypothesized to reflect a reinforcement of NAO-like atmospheric circulation variability, possibly driven by more vigorous changes in North Atlantic deep-water formation.

The timing of the flooding of the Danish/German/Swedish straits and the first marine influence in the Baltic Basin after the Ancylus Lake stage has been much debated. Here we present 47 new radiocarbon ages from sediment cores retrieved from Danish waters, ages that record the marine transgression of the area. Most of the ages are based on shells of marine molluscs, hence the ages are uncertain because we do not know the precise reservoir age in the past.We use a reservoir age of 400 years, which is based on dating of museum specimens collected live before testing of nuclear bombs. It appears from the ages that the Great Belt (Storebælt) was inundated before The Sound (Øresund). The oldest ages on marine shells from the northern part of the Great Belt region are about 8.9 cal. ka BP.From the central part of the Great Belt, the oldest ages are about 8.2 cal. ka BP, in Mecklenburg Bay 8.1 cal. ka BP and in the Arkona Basin 7.1 cal. ka BP. Sediments deposited prior to the occurrence of marine molluscs contain brackish-water ostracods (Cyprideis torosa and Cytheromorpha fuscata); the sediments are usually laminated and non-bioturbated mud. The brackish-water phase may have lasted 500–1000 years. The youngest lake deposits without signs of marine influence are dated to c. 8.8 cal. ka BP(central Great Belt), 8.7 cal. ka BP(Little Belt)and8.5 cal. ka BP(Arkona Basin). We see no evidence for marine influence at c. 10 cal. ka BP as recorded by some studies from the Baltic Basin.

Hydroxylated glycerol dibiphytanyl glycerol tetraethers (OH-GDGTs) produced by both marine and freshwater thaumarchaea are increasingly used for the reconstruction of past sea surface temperature (SST). They occur throughout the modern Baltic Sea, but it is unknown if their OH-GDGTs can be used for assessing past SST in this area, where salinity has changed considerably over the Holocene. Three commonly applied OH-GDGT for SST reconstruction, i.e., the OH-GDGT%, RI-OH, and RI-OH´ indices, were tested using a Thaumarchaeotal culture enriched from the Baltic Sea grown at 4 and 22 °C, and 12 surface sediments from the Baltic Sea and the adjacent Skagerrak. In the culture experiments all three proxies showed the expected response with the rise of temperature, but their absolute values were not always in line with existing marine core top calibrations, especially for the OH-GDGT% index. Of the two proxies based on the distribution of OH-GDGTs, the RI-OH index shows no increase with increasing mean annual SST, whilst the RI-OH´ index shows a linear correlation with SST when the sediments from the Bothnian Sea and Bay area, with a reduced salinity and increased lateral sediment influx, are omitted. Two sedimentary Holocene records from the Arkona and Gotland basins were studied, the latter in high resolution. In the brackish phase of the Baltic Sea (the Littorina Sea stage), the RI-OH´ index shows a good correlation with the TEX86L, an established temperature proxy in the Baltic Sea, and can be used to identify important climatic events. However, during the preceding Ancylus Lake phase the RI-OH´ (and RI-OH) index records far too high values, resulting in anomalously high SST estimates. This is probably because freshwater thaumarchaea adjust their OH-GDGTs differently, as has been shown for thaumarchaea in Lake Lugano’s water column. In the Littorina Sea, the Ancylus Lake and the Yoldia Lake phases of the Baltic basin, the record of the RI-OH´ index, thus, most likely reflects both changes in temperature and salinity. Overall, our study indicates that a reduced salinity increases the values of the RI-OH´ (and RI-OH) indices substantially and this should be considered when applying these proxies in other settings.

Northern and southern hemispheric influences—particularly changes in Southern Hemisphere westerly winds (SSW) and Southern Ocean ventilation—triggered the stepwise atmospheric CO2increase that accompanied the last deglaciation. One approach for gaining potential insights into past changes in SWW/CO2upwelling is to reconstruct the positions of the northern oceanic fronts associated with the Antarctic Circumpolar Current. Using two deep-sea cores located ~600 km apart off the southern coast of Australia, we detail oceanic changes from ~23 to 6 ka using foraminifer faunal and biomarker alkenone records. Our results indicate a tight coupling between hydrographic and related frontal displacements offshore South Australia (and by analogy, possibly the entire Southern Ocean) and Northern Hemisphere (NH) climate that may help confirm previous hypotheses that the westerlies play a critical role in modulating CO2uptake and release from the Southern Ocean on millennial and potentially even centennial timescales. The intensity and extent of the northward displacements of the Subtropical Front following well-known NH cold events seem to decrease with progressing NH ice sheet deglaciation and parallel a weakening NH temperature response and amplitude of Intertropical Convergence Zone shifts. In addition, an exceptional poleward shift of Southern Hemisphere fronts occurs during the NH Heinrich Stadial 1. This event was likely facilitated by the NH ice maximum and acted as a coup-de-grâce for glacial ocean stratification and its high CO2capacitance. Thus, through its influence on the global atmosphere and on ocean mixing, “excessive” NH glaciation could have triggered its own demise by facilitating the destratification of the glacial ocean CO2state.

Long-chain alkenones (LCAs) and alkenoates (LCEs) are highly valuable biomarkers for paleotemperature reconstructions. A major problem, however, for accurate quantification of these compounds using gas chromatography (GC) is co-elution with steryl ethers, wax esters, saturated ketones and other numerous mid-polarity compounds frequently encountered in marginal marine and lake sediments. Co-elution during GC separation is prevalent, particularly if the full homologous series of alkenones and alkenoates are to be analyzed. Taking advantage of the presence of two or more double bonds in LCAs and LCEs, the conventional silica gel impregnated with silver nitrate has previously been used to remove co-eluting compounds for LCAs. However, this conventional argentation chromatography is hampered by the extreme instability of silver nitrate, poor reproducibility, low recovery and short lifetime. Here we demonstrate a highly efficient flash chromatographic approach based on silver thiolate chromatographic material (AgTCM) that overcomes the shortcomings of the traditional argentation chromatography and allows repeated sample preparation (up to 62 samples in one test) with little loss in separation efficiency. AgTCM selectively extracts LCAs and LCEs and effectively eliminates co-eluting compounds including steryl ethers and wax esters for the subsequent gas chromatography (GC) analysis. This new method, therefore, allows low-cost and high-throughput sample preparation for comprehensive quantification of the full homologous series of LCAs and LCEs in marine and lake sediments.

In shallow lakes, increasing phosphorus (P) loading has often been accompanied by a shift from a clear-water, macrophyte-dominated state to a turbid state featuring phytoplankton dominance. The effect of a regime shift on P burial and P fractions in lake sediments, however, is poorly understood. We used sediment cores from a eutrophic hard-water lake (Lake Gollinsee, Germany) that had undergone a regime shift (in approximately 1917) to investigate the effect on the accumulation rate of P and on changes in P forms. The cores were dated using Hg contents and radioisotopes ( 210 Pb, 137 Cs, and 241 Am). A combination of total organic carbon to total nitrogen ratios (TOC:TN), δ 13 TOC values, X-ray fluorescence calcium (Ca) counts, and sediment colour clearly distinguished sediment layers that were deposited during periods of macrophyte or phytoplankton dominance. The accumulation rate of total P (TP) in the sediments was 1.8 times higher after the regime shift and was associated with changes in the distribution of P fractions. The proportions of loosely-(NH 4 Cl-extracted TP) and Ca-(HCl-extracted TP) bound P decreased significantly, whilst the proportions of biogenic P (NaOH-extracted NRP) and aluminium-bound P (NaOH-extracted SRP) increased significantly. A higher dry mass deposition rate, reduced burial of stable Ca-P complexes, and increased contents and proportions of the mobile iron-bound (BD-extracted TP) and biogenic P fractions in the near-surface sediment layers are assumed to have enhanced the internal cycling of P and hence to have helped to maintain a state of phytoplankton dominance.

The northern sector of the Greenland ice sheet is considered to be particularly susceptible to ice mass loss arising from increased glacier discharge in the coming decades. However, the past extent and dynamics of outlet glaciers in this region, and hence their vulnerability to climate change, are poorly documented. In the summer of 2019, the Swedish icebreaker Oden entered the previously unchartered waters of Sherard Osborn Fjord, where Ryder Glacier drains approximately 2% of Greenland’s ice sheet into the Lincoln Sea. Here we reconstruct the Holocene dynamics of Ryder Glacier and its ice tongue by combining radiocarbon dating with sedimentary facies analyses along a 45 km transect of marine sediment cores collected between the modern ice tongue margin and the mouth of the fjord. The results illustrate that Ryder Glacier retreated from a grounded position at the fjord mouth during the Early Holocene (>10.7 ± 0.4 cal ka BP) and receded more than 120 km to the end of Sherard Osborn Fjord by the Middle Holocene (6.3 ± 0.3 cal ka BP), likely becoming completely land-based. A re-advance of Ryder Glacier occurred in the Late Holocene, becoming marine-based around 3.9 ± 0.4 cal ka BP. An ice tongue, similar in extent to its current position was established in the Late Holocene (between 3.6 ± 0.4 and 2.9 ± 0.4 cal ka BP) and extended to its maximum historical position near the fjord mouth around 0.9 ± 0.3 cal ka BP. Laminated, clast-poor sediments were deposited during the entire retreat and regrowth phases, suggesting the persistence of an ice tongue that only collapsed when the glacier retreated behind a prominent topographic high at the landward end of the fjord. Sherard Osborn Fjord narrows inland, is constrained by steep-sided cliffs, contains a number of bathymetric pinning points that also shield the modern ice tongue and grounding zone from warm Atlantic waters, and has a shallowing inland sub-ice topography. These features are conducive to glacier stability and can explain the persistence of Ryder’s ice tongue while the glacier remained marine-based. However, the physiography of the fjord did not halt the dramatic retreat of Ryder Glacier under the relatively mild changes in climate forcing during the Holocene. Presently, Ryder Glacier is grounded more than 40 km seaward of its inferred position during the Middle Holocene, highlighting the potential for substantial retreat in response to ongoing climate change. The northern sector of the Greenland ice sheet is considered to be particularly susceptible to ice mass loss arising from increased glacier discharge in the coming decades. However, the past extent and dynamics of outlet glaciers in this region, and hence their vulnerability to climate change, are poorly documented. In the summer of 2019, the Swedish icebreaker Oden entered the previously unchartered waters of Sherard Osborn Fjord, where Ryder Glacier drains approximately 2% of Greenland’s ice sheet into the Lincoln Sea. Here we reconstruct the Holocene dynamics of Ryder Glacier and its ice tongue by combining radiocarbon dating with sedimentary facies analyses along a 45 km transect of marine sediment cores collected between the modern ice tongue margin and the mouth of the fjord. The results illustrate that Ryder Glacier retreated from a grounded position at the fjord mouth during the Early Holocene (>10.7 ± 0.4 cal ka BP) and receded more than 120 km to the end of Sherard Osborn Fjord by the Middle Holocene (6.3 ± 0.3 cal ka BP), likely becoming completely land-based. A re-advance of Ryder Glacier occurred in the Late Holocene, becoming marine-based around 3.9 ± 0.4 cal ka BP. An ice tongue, similar in extent to its current position was established in the Late Holocene (between 3.6 ± 0.4 and 2.9 ± 0.4 cal ka BP) and extended to its maximum historical position near the fjord mouth around 0.9 ± 0.3 cal ka BP. Laminated, clast-poor sediments were deposited during the entire retreat and regrowth phases, suggesting the persistence of an ice tongue that only collapsed when the glacier retreated behind a prominent topographic high at the landward end of the fjord. Sherard Osborn Fjord narrows inland, is constrained by steep-sided cliffs, contains a number of bathymetric pinning points that also shield the modern ice tongue and grounding zone from warm Atlantic waters, and has a shallowing inland sub-ice topography. These features are conducive to glacier stability and can explain the persistence of Ryder’s ice tongue while the glacier remained marine-based. However, the physiography of the fjord did not halt the dramatic retreat of Ryder Glacier under the relatively mild changes in climate forcing during the Holocene. Presently, Ryder Glacier is grounded more than 40 km seaward of its inferred position during the Middle Holocene, highlighting the potential for substantial retreat in response to ongoing climate change.

The Quaternary is well known for being a period of the geological record that saw significant and alternating climatic fluctuations. Here, we concentrate on the last 94 millennia that saw Australia and its surrounding seas undergo significant environmental changes. Importantly also it is during that this period of time includes the arrival and settlement of humans in Australia. Following on from Marine Isotopic Stage 4 (MIS4) that saw, over the period of 71-59 ka BP, a significant eustatic sea level drop (similar to 100 m), low sea-surface temperatures as well as glacial conditions on land, Australia experienced wet conditions again, but eventually progressively entered into a glacial phase. By then, humans had arrived on this large continent and later the megafauna progressively became extinct. This paper describes in detail the climate over this period, based principally on the high-resolution record of two marine sediment cores located offshore of the southern Australian margin, and that are then compared with known events on land as well as other deep-sea cores. Particular emphasis is placed on the period that spans the Last Glacial Maximum (LGM), the extent of which thus far had been poorly defined in the Australian region. Emphasis is placed on the period from 34 to 14 ka to demonstrate that (1) the LGM, defined here as the period 24 to 18 ka, was not always extremely dry and cold, and (2) that people were able to live in inland Australia as water existed in places, despite generally cold conditions. We also show using a series of ten maps-at 2 ka intervals over the 34-14 ka period-the waxing and waning of oceanic fronts such as the Subtropical and Subantarctic Fronts, link sea-surface temperatures (SST) with periods of glacial expansion in the Australian Alps and Tasmania, as well as the South Island of New Zealand, and the extent of the Leeuwin Current down to south of Australia. Aeolian dust was transported at sea over the main core site from similar to 26 to 18 ka period, pointing to arid conditions in Australia, but this period was punctuated by two significant reductions in dust transport to both core sites, coinciding with a retreat of the Subantarctic Front away from the Australian southern coastline, and slight SST shifts for a few centuries at similar to 24 and 22-21 ka BP. At the peak of the cold phase, from 23 to 18.4 ka, summer SST were of the order of 9 degrees C below that experienced during the middle of the Holocene at about 6 ka BP. A similar temperature drop was also experienced offshore the northwestern corner of Western Australia. Periglacial activity dates for the Australian mainland and Tasmania pool around similar to 22 ka. On either side of the 24 ka event, the chain of lakes in the Willandra Lakes retained water and human activity was registered in the area. Elsewhere in Australia, water was present to sustain human populations during the cold and dry times. It is postulated here that northern Australia was at times quite wet, especially during MIS4. The cessation of the LGM is set on 18.4 ka from offshore southern Australia as confirmed for the deepsea cores by several proxies, and more likely at 18 ka inland. Finally, we document that it took two millennia for "glacial" conditions to end in the Australian region as registered in our deep-sea core records, well after the lowest global sea level similar to 20.6 and 20 ka, respectively, started to rise. There is also evidence of another very cold period determined on the Australian mainland, Tasmania and the South Island of New Zealand centred around 27 ka, after which time warmer conditions occurred before the start of the LGM. This is matched with our deep-sea records with very cold conditions and changes in oceanographic conditions. (C) 2020 Elsevier Ltd. All rights reserved.

The Baltic Sea is a semi-enclosed, brackish water sea in northern Europe. The deep basins of the central Baltic Sea regularly show hypoxic conditions. In contrast, the northern parts of the Baltic Sea, the Bothnian Sea and Bothnian Bay, are well oxygenated. Lateral inflows or a ventilation due to convection are possible mechanisms for high oxygen concentrations in the deep water of the northern Baltic Sea. In March 2017, conductivity–temperature–depth (CTD) profiles and bottle samples, ice core samples, and brine were collected in the Bothnian Bay. In addition to hydrographic standard parameters, light absorption has been measured in all samples. A complementary numerical model simulation provides quantitative estimates of the spread of newly formed bottom water. The model uses passive and age tracers to identify and trace different water masses. Observations indicate a recent ventilation of the deep bottom water at one of the observed stations. The analysis of observations and model simulations shows that the Bothnian Bay is ventilated by dense water formed due to mixing of Bothnian Sea and Bothnian Bay surface water initializing lateral inflows. The observations show the beginning of the inflow and the model simulation demonstrates the further northward spreading of bottom water. These events occur during wintertime when the water temperature is low. Brine rejected during ice formation barely contributes to dense bottom water.

A high-resolution marine sediment core NP16-Kro1-MCB from Krossfjorden, Western Svalbard is studied to investigate changes in sea surface conditions in the fjord during the last 60 years (1953-2014). The diatom-based reconstruction of August sea surface temperature (aSST) demonstrates a clear warming trend of 0.6 °C through the record. As inferred from Marginal Ice Zone (MIZ) diatoms, surface warming occurs in parallel with a decline in sea ice extent (SIE) during recent decades. Factor analysis identified variations in diatom assemblages representing different water masses, showing a dominance of Arctic water diatoms throughout the period and decadal variations in the sea ice assemblage during periods of peak sea ice extent. The strong dominance of Arctic water diatoms along with increasing aSST suggest prolonged open water conditions and increased sea ice melting in the region throughout the observed period. The reconstructed ocean surface changes are in line with the background warming occurring over the Arctic region. A comparison with instrumental records from neighboring regions supports the quality of the reconstructions, including the average reconstructed aSST and the magnitude of the warming trend. We suggest that increased CO2 forcing together with ocean-atmospheric interaction have caused the increasing SST trend and decreasing sea ice presence in Krossfjorden rather than an increasing influence from Atlantic Water, which has amplified changes in many regions of Svalbard.

Deep water renewal and ventilation of the Baltic Sea is commonly considered to occur solely via saltwater inflows from the North Sea. However, recent analysis of geophysical and sediment proxy data suggests convective deep water formation during wintertime as a second process that has contributed to the ventilation of the northern and central Baltic Sea bottom waters during cold climate periods. Here, we investigate the role of deep water formation in the northern Baltic Sea in a regional ocean circulation model. Selecting the particularly cold winter 1986/1987 as a reference period, we perform sensitivity experiments in which the atmospheric temperature forcing is changed and/or localized brine rejection on subgrid scales is accounted for through a parameterization. We study the sinking and circulation of water masses via passive tracers introduced into the model. Generally, our model results support the established view that convective deep water formation does not play a major role in Baltic Sea deep water renewal. While a reduction of air temperatures does not qualitatively change the sinking and circulation of water masses in our experiments, brine rejection could potentially lead to localized deep water formation. However, the impact is too weak to possibly change the large-scale deep water exchange between the Baltic proper and the northern Baltic sub-basins. Although being in line with established knowledge, through consideration of the model limitations, our results provide insights on how and under which circumstances convective deep water formation could potentially have occurred during cold climate periods. ©2020. The Authors.

Numerous hydro-acoustic studies of the seabed of the Baltic Sea have revealed the unusual occurrence of sediment contourite drifts and re-suspension at greater water depths. In addition, radiocarbon dating of bulk sediments indicates significant age reversals. We present new geophysical, sediment proxy data (including extensive radiocarbon dating) and hydrographic measurements, which are combined with results of numerous marine geological studies performed during the last decades. These data indicate that a deep-water formation process significantly affected the seabed dynamics during regional climatically cold phases during the last c. 7,000 years. We propose that, during the colder periods (e.g. the Little Ice Age), newly formed bottom waters likely caused widespread re-suspension of organic carbon-rich laminated sediments that were deposited during the preceding warm periods in shallower areas, and this material was transported to and re-deposited in the deeper parts of the Baltic Sea sub-basins. In our scenario, a topographic feature, known as the Baltic Sea Klint, acted as a hydrographic barrier for deep-water formed in the northern Baltic. Thus, during the cold periods increased lateral matter influx from the northern Baltic led to the accumulation of much thicker macroscopically homogenous clayey sediments in sub-basins north of the Klint. Moreover, deep-water formation produced bottom currents that led to the formation of sediment contourite drifts at water depths of >200 m in the Bothnian Sea, the Aland Deep and northern central Baltic Sea sub-basins. Bottom water ventilation in the Baltic Sea is generally assumed to be determined solely by the inflow of oxygen-rich, saline water from the North Sea, but we challenge this assumption and postulate that deep-water formation is a key process that ventilates the bottom waters of the Baltic Sea during climatically cold periods with substantial implications for its sedimentary archive.

A multidisciplinary investigation performed on gravity cores, supplemented by a new radiocarbon dating of macrofossils, outlined the environmental evolution of Arkona Basin in the Holocene. Studies comprising the analysis of diatoms, geochemistry, grain size, and radiocarbon dating clarified the knowledge about the age and environmental changes during freshwater and marine transgressions and regressions. The acoustic profiling method allowed proper interpretation of the analysis results from sediment cores and created a proper lithostratigraphic model of the investigated basin. Baltic Ice Lake and Yoldia Sea sediment layers were identified acoustically and cores penetrated the final regression surface of the Baltic Ice Lake. The analyses of cores concentrated on Ancylus Lake and Littorina Sea sediments. Two phases of the Ancylus Lake stage were identified. The peaks of loss on ignition in the cores, observed as a strong acoustic reflector, were identified as a marker of Ancylus Lake regression between the first, high-water level phase and the second, shallow-water phase. The age of onset of Littorina transgression was estimated at around 7000–7200 cal BP and was younger than some previous estimations. The retrieved age confirmed the suggestion that the marine environment appeared first in Mecklenburg Bay and then after a few hundred years in Arkona Basin as an intensive transgression. The appearance of Littorina Sea revealed a fast shift of the environment from lacustrine to marine. The first phase of the Littorina stage was recognised as a rapidly increasing water level and high-energy environment; the second showed rapidly increasing salinity in the calmer conditions of sedimentation.

Past sea ice conditions and open water phytoplankton production were reconstructed from a sediment core taken in Disko Bugt, West Greenland, using the sea ice biomarker IP25 and other specific phytoplankton biomarker (i.e., brassicasterol, dinosterol, HBI III) records. Our biomarker record indicates that Disko Bugt experienced a gradual expansion of seasonal sea ice during the last 2.2 kyr. Maximum sea ice extent was reached during the Little Ice Age around 0.2 kyr BP. Superimposed on this longer term trend, we find short-term oscillations in open water primary production and terrigenous input, which may be related to the Atlantic Multidecadal Oscillation and solar activity changes as potential climatic trigger mechanisms. A direct sample-to-sample multiproxy comparison of our new biomarker record with microfossil (i.e., benthic foraminifera, dinocysts, and diatoms) and other geochemical records (i.e., alkenone biomarkers) indicates that different proxies are influenced by the complex environmental system with pronounced seasonal changes and strong oceanographic gradients, e.g., freshwater inflow from the Greenland Ice Sheet. Differences in sea ice reconstructions may indicate that the IP25 record reflects only the relatively short sea ice season (spring), whereas other microfossil reconstructions may reflect a longer (spring–autumn) interval.

The sediments of the Landsort Deep and Gotland Basin in the central Baltic Sea are strongly enriched in Mn carbonate. However, conceptual models attempting to explain the intense Mn carbonate precipitation in both basins are in part conflicting. In the Gotland Basin model, deposited Mn oxides are converted to Mn carbonate after the oxygenation of euxinic bottom waters enriched in dissolved Mn by major Baltic inflows (Huckriede and Meischner, 1996). By contrast, according to the Landsort Deep model, Mn carbonate precipitation occurs independent of oxygenation events below a euxinic water column (Lepland and Stevens, 1998; Lenz et al., 2015). In this study, we investigated Mn solid-phase signatures in recent/sub-recent well-dated sediments from the Landsort Deep and compared them to long-term observations to identify the hydrographic conditions favoring Mn carbonate formation. The comparisons of water column O2 and sulfide time series with sedimentary Mn carbonate enrichments identified long-lasting bottom water oxygenation as an important environmental factor in the enhancement of Mn carbonate precipitation (up to 32 wt% Mn) in the Landsort Deep. Thus, by preventing the escape of dissolved Mn from still reducing sediments into the open water column, these conditions allow the accumulation of large amounts of Mn-oxide particles at the sediment/water interface, with their subsequent conversion to Mn carbonate. The euxinic conditions that have prevailed almost continuously in the Landsort Deep since roughly AD 2000 do not favor Mn enrichment (maximum 0.9 wt% Mn), highlighting the importance of the recurring oxygenation of bottom water, most likely via medium-intensity inflows. The Mn abundances in seven sediment cores from water depths of 190–437 m together with Mn balance calculations indicated that the Mn inventory in the water column in response to porewater Mn reflux and detrital Mn input is sufficient for Mn carbonate enrichment only in the deepest part of the basin.

The Leeuwin Current (LC), an eastern boundary current, transports tropical waters from the Indo-Pacific Warm Pool (IPWP) towards southern latitudes and modulates oceanic conditions offshore southern Australia. New, high-resolution planktic foraminifer assemblage data and alkenone-derived sea surface temperatures (SST) provide an in-depth view on LC variability and mechanisms driving the current's properties during the mid to late Holocene (last c. 7.4 ka BP). Our marine reconstructions highlight a longer-term mid to late Holocene reduction of tropical heat export from the IPWP area into the LC. Mid Holocene (c. 7.4 to 3.5 ka BP) occurrence of high SSTs (>19.5 °C), tropical planktic foraminifera and a well-stratified water column document an enhanced heat export from the tropics. From c. 3.5 ka BP onwards, a weaker LC and a notably reduced tropical heat export cause oceanic cooling offshore southern Australia. The observed mid to late Holocene trends likely result from large-scale changes in the IPWP's heat storage linked to the El Nino-Southern Oscillation (ENSO) phenomenon. We propose that a strong and warm LC occurs in response to a La Nina-like state of ENSO during the mid Holocene. The late Holocene LC cooling, however, results from a shift towards an El Nino-like state and a more variable ENSO system that causes cooling of the IPWP. Superimposed on these longer-term trends we find evidence of distinct late Holocene millennial-scale phases of enhanced El Nino/La Nina development, which appear synchronous with northern hemispheric climatic variability. Phases of dominant El Nino-like states occur parallel to North Atlantic cold phases: the ‘2800 years BP cooling event’, the ‘Dark Ages’ and the ‘Little Ice Age’, whereas the ‘Roman Warm Period’ and the ‘Medieval Climate Anomaly’ parallel periods of a predominant La Nina-like state. Our findings provide further evidence of coherent interhemispheric climatic and oceanic conditions during the mid to late Holocene, suggesting ENSO as a potential mediator.

Arctic sea ice is a critical component of the climate system as it influences the albedo, heat, moisture and gas exchange between ocean and atmosphere as well as the ocean's salinity. An ideal location to study natural sea ice variability during pre-industrial times is the East Greenland Shelf that underlies the East Greenland Current (EGC), the main route of Arctic sea ice and freshwaters from the Arctic Ocean into the northern North Atlantic. Here, we present a new high-resolution biomarker record from the East Greenland Shelf (73°N), which provides new insights into the sea ice variability and accompanying phytoplankton productivity over the past 5.2 kyr. Our IP25 based sea ice reconstructions and the inferred PIP25 index do not reflect the wide-spread late Holocene Neoglacial cooling trend that follows the decreasing solar insolation pattern, which we relate to the strong influence of the polar EGC on the East Greenland Shelf and interactions with the adjacent fjord throughout the studied time interval. However, our reconstructions reveal several oscillations with increasing/decreasing sea ice concentrations that are linked to the known late Holocene climate cold/warm phases, i.e. the Roman Warm Period, Dark Ages Cold Period, Medieval Climate Anomaly and Little Ice Age. The observed changes seem to be connected to general ocean atmosphere circulation changes, possibly related to North Atlantic Oscillation and Atlantic Multidecadal Oscillation regimes. Furthermore, we identify a cyclicity of 73–74 years in sea ice algae and phytoplankton productivity over the last 1.2 kyr, which may indicate a connection to Atlantic Multidecadal Oscillation mechanisms.

The Baltic Sea (~393 000 km2) is the largest brackish sea in the world and its hydrographic and environmental conditions are strongly dependent on the frequency of saline water inflows from the North Sea. To improve our understanding of the natural variability of the Baltic Sea ecosystem detailed reconstructions of past saline water inflow changes based on palaeoecological archives are needed. Here we present a high-resolution study of benthic foraminiferal assemblages accompanied by sediment geochemistry (loss on ignition, total organic carbon) and other microfossil data (ostracods and cladocerans) from a well-dated 8-m-long gravity core taken in the Bornholm Basin. The foraminiferal diversity in the core is low and dominated by species of Elphidium. The benthic foraminiferal faunas in the central Baltic require oxic bottom water conditions and salinities >11–12 PSU. Consequently, shell abundance peaks in the record reflect frequent saline water inflow phases. The first appearance of foraminiferal tests and ostracods in the investigated sediment core is dated to c. 6.9 cal. ka BP and attributed to the first inflows of saline and oxygenated bottom waters into the Bornholm Basin during the Littorina Sea transgression. The transgression terminated the Ancylus Lake phase, reflected in the studied record by abundant cladocerans. High absolute foraminiferal abundances are found within two time intervals: (i) c. 5.5–4.0 cal. ka BP (Holocene Thermal Maximum) and (ii) c. 1.3–0.75 cal. ka BP (Medieval Climate Anomaly). Our data also show three intervals of absent or low saline water inflows: (i) c. 6.5–6.0 cal. ka BP, (ii) c. 3.0–2.3 cal. ka BP and (iii) c. 0.5–0.1 cal. ka BP (Little Ice Age). Our study demonstrates a strong effect of saline and well-oxygenated water inflows from the Atlantic Ocean on the Baltic Sea ecosystem over millennial time scales, which is linked to the major climate transitions over the last 7 ka.

To detect climatic linkages between the Baltic Sea, the Skagerrak and the Nordic Seas, we present multi-proxy reconstructions covering the last 4500 years from three sediment cores taken in the Skagerrak and along the SW Norwegian margin. Foraminiferal assemblages at all three sites show a distinct change at c. 1700 years BP, associated with a transition from absence and rare occurrence of Brizalina skagerrakensis during c. 4500–2300 years BP to its subsequent abundance increase, suggesting a stronger influence of nutrient-rich water-masses during the last c. 1700 years. Increased nutrient availability, which probably stimulated higher primary productivity, is further supported by an increase in diatoms, total organic carbon and benthic foraminiferal species indicative of high productivity and carbon fluxes during the last c. 1700 years as compared to c. 4500–2300 years BP. The amplitude of the B. skagerrakensis signal is largest in the central Skagerrak and gradually becomes smaller towards the Norwegian Sea suggesting that the dominant source of the nutrient-rich water was the brackish outflow from the Baltic Sea. The generally lower abundances of planktonic foraminifera since c. 1700 years BP support the hypothesis of less saline surface water conditions in the Skagerrak. These results agree with other studies, which suggest a stronger Baltic outflow over the last 1700 years coinciding with a general cooling, increased wintertime westerlies bringing more winter precipitation to northern Europe, increased river runoff and higher frequency of floods. The increase in outflow also occurs during deposition of laminated sediments in the deep Baltic Sea. Leakage of dissolved inorganic phosphorus from anoxic sediments, as well as enhanced erosion due to deforestation in combination with higher runoff from Norway, coastal upwelling and more vigorous frontal dynamics may all have contributed to higher nutrient availability within the adjacent Skagerrak during the last 1700 years BP as compared to c. 4500–2300 years BP, when low productivity prevailed in the study area.

International audience; In Europe, the transition from hunter-gatherer-fisher-based communities into societies mainly relying on farming was spread from the southeast towards the north and west during the Holocene. In central Germany, farming was adopted at similar to 7500 cal. yr BP, whereas the shift is evident at similar to 6000-5500 cal. yr BP in northern Germany and southern Scandinavia. Consequently, farming techniques were available for more than a millennium. Some studies argue that climate change might have played a role in the onset of farming in those areas. The aim of this study is to reconstruct the mid-to late-Holocene sea surface temperature (SST) evolution in the Skagerrak to document potential regional climatic impacts on changes in human economy. We compare our results with a record of human settlement activity in northern Germany and southern Scandinavia. Prior to similar to 6300 cal. yr BP, warm SSTs are documented throughout the Skagerrak, suggesting dominance of North Atlantic sourced water inflow providing mild climatic conditions. Between similar to 6300 and 5400 cal. yr BP, that is, concomitant with the shift in human economy, SSTs in the NE Skagerrak dropped by similar to 5-6 degrees C, as also documented in mean annual air temperatures in central South Sweden, although less pronounced. The regional cooling suggests outflow of colder Baltic Sea water only affecting the NE Skagerrak and central South Sweden. Probably, numerous severe winters reflecting a continental-dominated atmospheric circulation pattern prevailed over the region. These changes most likely caused a gradual restriction in natural food sources, in particular from the marine realm. We thus suggest that hunter-gatherer-fishers were forced to adopt farming strategies to counter-balance this environmental stress. Our results indicate that regional changes in oceanography probably amplifying North Atlantic climate change in the western Baltic were an important factor that played a role in the adoption of farming in northern Germany and southern Scandinavia.

Reconstructions of environmental changes at sub-decadal to decadal resolution based on central Baltic Sea sediments rely on accurate and precise high-resolution sediment depth/age relationships. A model chronology for Baltic Sea sediments is presented here based on established historical records of anthropogenic radionuclides (137Cs/241Am/bomb14C), polychlorinated biphenyls (PCBs), lead (Pb) and stable lead isotope (206/207Pb ratios), and radionuclide 210Pb and 14C decay dating methods. Marker horizons consisting of chemical precipitates formed by documented Major Baltic Inflow (MBIs) events and an extended diatom bloom period were also integrated into the model. The main time markers in Baltic Sea sediments that formed during the last 120 years were the following: (i) the deepest observation of 210Pbunsupp. (marking the 210Pb dating horizon) and departure of Hg from natural background levels at c. AD 1900; (ii) first detectable presence of PCBs at AD 1935; (iii) radionuclide production (i.e. 241Am) due to nuclear weapons testing between AD 1954 and AD 1975, with a peak in AD 1963; (iv) maximum heavy metal and PCB concentrations in the AD 1960s/1970s; (v) the Chernobyl nuclear accident in AD 1986 as a sharp 137Cs increase; (vi) exceptionally strong diatom blooms with a massive diatom layer found in the Eastern Gotland Basin in AD 1988–1990; and (vii) characteristic manganese-carbonate layers in the deeper central basins formed by MBIs in AD 1993 and AD 2003. A precise and accurate sediment depth/age relationship can only be achieved in restricted areas of the Baltic Sea where continuous sedimentation has prevailed and there has been limited postdepositional disturbance. We demonstrate that parallel Hg and 137Cs measurements can be used to assess the quality of sediment sequences for high-resolution environmental reconstructions. We show examples of sediment profiles that conform to the historical record, and examples from Western Baltic Sea areas where it appears to be impossible to establish accurate geochronologies.

Summer cyanobacterial blooms represent a threat for the Baltic Sea ecosystem, causing deoxygenation of the bottom water and the spread of the so-called dead zones. The time history of the Baltic Sea cyanobacterial blooms is known from in situ and satellite observations since the early 1980s, but still not well understood. By comparing both weekly-resolved trap sediments and a well-dated sediment core from the Eastern Gotland Basin with monitoring and satellite cyanobacterial data of the last ca. 35 years, it is shown here that 6- and 7-methylheptadecane lipids (expressed as 6+7Me-C17:0) are robust semi-quantitative biomarkers for diazotrophic cyanobacteria, and likely mainly for Nodularia spumigena. Using this organic proxy, it was thus possible to reconstruct the history of cyanobacterial blooms beyond the observational period with a resolution of 2–4 years since 1860. Cyanobacteria were constantly present, but in relatively low abundance until 1920, when they started to alternate between periods with high and low abundance. Interestingly, there seems to be no significant increase in cyanobacterial abundance in the 1950s, when eutrophication and deoxygenation of the Baltic Sea increased considerably. Decadal to multi-decadal fluctuations are likely rather related to variability in the Baltic Sea surface temperature and, ultimately, to the Atlantic Multidecadal Oscillation. A 7000 years long 6+7Me-C17:0 record from the Bothnian Sea also suggests a relationship with the mean summer temperature in the Baltic Sea region, but at a multi-centennial to multi-millennial timescale. The intensity of the cyanobacterial blooms in the Baltic Sea is thus likely mainly related to natural processes such as temperature variability, at least at a multi-decadal to multi-millennial timescale.

We present a record of dinoflagellate cyst assemblage composition, abundance, and morphology from the central Baltic Sea, spanning the late 19th to the early 21st centuries. Environmental time-series were analyzed in relation to changes in community structure and diversity as inferred from the sediment record, and average summer sea surface salinity (SSS) was reconstructed based on the average process length of Protoceratium reticulatum resting cysts. The reconstructed summer SSS was compared to instrumental data for a critical evaluation of this approach. The most abundant species in this record were P. reticulatum and Biecheleria baltica, and on average ten taxa were identified per sample. The cyst record of B. baltica indicated that although this species has been present in the Gotland Basin at least since the 1880s, its concentrations have increased significantly since the 1980s, possibly linked to eutrophication. Variations in assemblage composition and P. reticulatum cyst morphology reflected patterns of major, instrumentally recorded hydrographic and environmental changes in the Baltic Sea during the past century. The variability in microfossil relative abundances was best explained by the average spring SSS as well as by the average NO3 concentrations during spring and by the combined effects of average summer SSS and NAO variability. Reconstructed summer SSS and instrumental SSS showed notable differences, depending on the year and function applied for reconstruction. Although roughly reflecting the same patterns, the reconstructed values are offset when compared to instrumental measurements. We put forward suggestions for improvement of the process-length method and recommend using the reconstructed values as an indication of relative changes in past summer sea surface salinity, preferably as part of a multiproxy approach.

The Northwest Atlantic is a region of major climate change over the twentieth century, affected by the weakening of the Atlantic meridional overturning circulation. To assess whether the ability of this region to absorb anthropogenic CO2 has been impacted by this change, we present the region\'s first long‐term carbon isotope (δ13C) time series of fossil foraminifera spanning the past 4,000 years. These records reveal an unprecedented negative δ13C excursion driven by anthropogenic CO2 penetration into the surface ocean, the “Suess effect” signal. This signal (amplitude −0.45‰) emerges in 1950 CE ± 15 with a decrease rate of 0.009 ± 0.001‰/yr. This marine signal is ~30% of the atmospheric Suess effect and emerges over a century later. Based on current estimates of the ratio of δ13CDIC change to dissolved inorganic carbon change and limited constraints on surface ocean residence times, we calculate a mean anthropogenic CO2 uptake rate of 0.6 ± 0.2 μmol/(kg yr) from 1950 to 2005.

A high-resolution marine sediment core NP16-Kro1-MCB from Krossfjorden, Western Svalbard is studied to investigate changes in sea surface conditions in the fjord during the last 60 years (1953–2014). The diatom-based reconstruction of August sea surface temperature (aSST) demonstrates a clear warming trend of 0.6 °C through the record. As inferred from Marginal Ice Zone (MIZ) diatoms, surface warming occurs in parallel with a decline in sea ice extent (SIE) during recent decades. Factor analysis identified variations in diatom assemblages representing different water masses, showing a dominance of Arctic water diatoms throughout the period and decadal variations in the sea ice assemblage during periods of peak sea ice extent. The strong dominance of Arctic water diatoms along with increasing aSST suggest prolonged open-water conditions and increased sea ice melting in the region throughout the observed period. The reconstructed ocean surface changes are in line with the background warming occurring over the Arctic region. A comparison with instrumental records from neighboring regions supports the quality of the reconstructions, including the average reconstructed aSST and the magnitude of the warming trend. We suggest that increased CO2 forcing together with ocean–atmospheric interaction have caused the increasing SST trend and decreasing sea ice presence in Krossfjorden rather than an increasing influence from Atlantic Water, which has amplified changes in many regions of Svalbard.

Evaluating and understanding bio-environmental relationships at different spatial and temporal scales is important for accurately assessing marine productivity in relation to oceanographic conditions in changing Arctic ecosystems. In this paper, we compare four different bio-environmental datasets, in order to shed new light on the complex marine ecosystem of the Baffin Bay-Labrador Sea region, as well as the historical links between environmental changes and the ecosystem. Satellite dataset was used to describe the regional distribution of chlorophyll a, sea ice concentration (SIC), sea surface temperature (SST) and sea surface salinity (SSS). Chlorophyll a data were also analysed together with shrimp assessment dataset to examine the relationship between marine pelagic productivity and bottom feeders off the Greenland coast. A microfossil dataset (diatom valves) from 57 surface sediment stations was used to describe diatom concentration and diversity across the study region. It was also used in combination with satellite data from the same sites to analyse species-environment relationships and develop a diatom-based transfer function for reconstructing past changes in SIC, SST and SSS. Subsequently, we applied the diatom transfer function on three short sediment cores from off SW and W Greenland spanning the last few centuries. A monitoring dataset of sea ice observations and instrumental measurements of water temperature and salinity, together with atmospheric data (air temperature and wind conditions) from Greenland, were used to develop a more complete picture of 20th and 21st century changes in ocean-climate conditions. Our comparative analyses show clear spatial distribution patterns of marine phytoplankton and identify key ecological groups linked to oceanic conditions. In addition, we find a significant correlation between the different trophic levels, culminating in a ‘productivity boom’ in 2001 CE. On a longer temporal scale, the transfer function reconstructions reveal a good correlation with the satellite, monitoring and observational trends, as well as a consistent, long-term decrease in sea ice linked to ocean-climate variability.

Mean body size decreases with increasing temperature in a variety of organisms. This size–temperature relationship has generally been tested through space but rarely through time. We analyzed the sedimentary archive of dinoflagellate cysts in a sediment record taken from the West Greenland shelf and show that mean cell size decreased at both intra‐ and interspecific scales in a period of relatively warm temperatures, compared with a period of relatively cold temperatures. We further show that intraspecific changes accounted for more than 70% of the change in community mean size, whereas shifts in species composition only accounted for about 30% of the observed change. Literature values on size ranges and midpoints for individual taxa were in several cases not representative for the measured sizes, although changes in community mean size, calculated from literature values, did capture the direction of change. While the results show that intraspecific variation is necessary to accurately estimate the magnitude of change in protist community mean size, it may be possible to investigate general patterns, that is relative size differences, using interspecific‐level estimates.

The Littorina Sea stage (past c. 7000 years) development of the northern, and its interactions with the central Baltic Sea have been influenced by spatially different but in the north very strong glacio-isostatic uplift. Here we investigate the impact of the isostatic readjustment on the northern Baltic Sea environment and on the water exchange with the central Baltic Sea using geochemical and microfossil records from well-dated sediment cores of the Bothnian Sea/Bay that are compared to sedimentary records of the Gotland Basin and Landsort Deep. Benthic foraminifera and manganese carbonate layers in the sediments from northern Baltic Sea sub-basins imply oxic marine intrusions from c. 7.0 cal. ka BP. Related water-column stratification together with enhanced primary productivity caused oxygen-depleted bottom waters and deposition of organic-rich laminated sediments during the Holocene Thermal Maximum. Comparable sedimentary/geochemical records in early Littorina Sea sediments of e.g. the eastern Gotland Basin therefore imply a low impact of isostatic uplift on the northern Baltic Sea environment. A climate-controlled decline in hypoxia induced a gradual deposition of rather homogenous sediments in the central and northern Baltic Sea during the mid-Littorina Sea stage. Progressive isostatic uplift caused a diverging environmental development in both basins during the mid- to late Littorina Sea stage. The Aland sill shoaled markedly allowing only strong inflows to enter the northern Baltic Sea as late as 2.3 cal. ka BP. The restricted deep-water exchange with the central Baltic Sea probably resulted in a pronounced freshening and oxic bottom waters. This, together with an uplift-related sediment re-deposition induced accumulation of rather homogenous sediments throughout the remaining Littorina Sea stage. By contrast, a climate-controlled depositional environment in the central Baltic Sea during this time caused the formation of homogeneous sediments during cold phases and laminated sediments during warm phases.

Many modern epicontinental seas were dry land before their marine flooding by the mid-Holocene glacioeustatic sea-level rise, whereas the Baltic Sea Basin was covered by a huge postglacial lake. This change from a postglacial lake to the present-day semi-enclosed brackish-water sea is studied here in sediment cores and acoustic profiles from the Baltic Sea major sub-basins, based on novel datasets combined with information extracted from earlier publications. In shallow areas ( 50m water depth), where it may be locally less clearly expressed due to reworking and bioturbation. Both in the shallow and deep areas, the brackish-water mud is strongly enriched in organic matter compared to underlying sediments. Bioturbation type changes at the flooding surface in response to the increased sedimentary organic content, but no firm-ground ichnofacies were developed because of low erosion. It is concluded that the base of the brackish-water mud is a robust allostratigraphic bounding surface that is identifiable by the lithologic examination of cores over the Baltic Sea. The surface is a distinct reflector in seismic-acoustic profiles, which facilitates mapping and basin-wide stratigraphic subdivision. Detailed geochronologic studies are required to confirm if sediments immediately overlying the erosional flooding surface in shallow areas are younger than the basal part of the brackish-water mud in deep areas that is predicted to be time-equivalent to the erosion.

To evaluate whether proxies that record surface, near-surface, and bottom water conditions from the North Iceland shelf have similar trends and periodicities, we examine Holocene century-scale paleoceanographic records from core MD99-2269. This core site lies close to the boundary between Atlantic and Arctic/Polar waters, and in an area frequently influenced by drift ice. The proxies are stable δ13C and δ18O values on planktonic and benthic foraminifera, alkenone-based sea-surface temperatures (SST°C), and foraminiferal Mg/Ca SST°C and bottom water temperature (BWT°C) estimates. These data were converted to equi-spaced 60-year time-series; significant trends were extracted using Singular Spectrum Analysis, which accounted for between 50% and 70% of the variance. In order to evaluate within-site ocean climate variability, a comparison between these data and previously published proxies from MD99-2269 was carried out on a standardized data set of 14 proxies covering the interval 400–9200 cal. yr BP. Principal component (PC) analysis indicated that the first two PC axes accounted for 57% of the variability with high loadings primarily defining ‘nutrient’ and ‘temperature’ proxies. Fuzzy k-mean clustering of the 14 climate proxies indicated major environmental changes at ~6350 and ~3450 cal. yr BP, which define local early-, middle-, and late-Holocene climatic shifts. Our results indicate that the major control on the combined proxy signal is the Holocene decrease in June insolation, but regional changes in such factors as sea-ice extent and salinity are required to explain the threefold division of the Holocene.

The Bornholm Basin has experienced a complex history linked closely to the structural development of the Tornquist Zone strike-slip fault system. The latest major tectonic development was late Cretaceous and early Tertiary Ronne Graben inversion caused by dextral transpressional strike-slip movements focused around rigid blocks such as Bornholm. By applying the strike-slip concept, it is possible to show that the Ronne Graben inversion push-up structure was created by restraining left step-over. The fault system continues northwest and north of Bornholm where the strike-slip movement resulted in right step-over and the formation of a pull-apart basin: the northern part of the Bornholm Basin. Detailed seismo-acoustic and sediment core studies show that, despite a rather uniform present bathymetry in the central part of the Bornholm Basin, the Quaternary deposits mirror the pull-apart basin synsedimentary depositional pattern with depocentres close to the major normal faults, bounding the Christianso Ridge. IODP Site M0065 penetrated a secondary fault zone and provided detailed information on neotectonic events and on environmental changes during the Holocene. The generally rather homogeneous sediments in the cores indicate a dominance of oxic conditions in the Bornholm Basin during the Holocene. However, strong internal seismic reflectors, indirectly reflecting variations in the sediment density and the reflection coefficient, were caused by a shift to stronger laminated, organic-rich sediments that were deposited during the Medieval Warm Period (MwP). The variable depth positions of the MwP-related strong reflectors indicate large differences in the spatial and temporal sedimentation pattern in the Bornholm Basin. These differences in the depositional environment may have been caused by local neotectonic events in the basin.