Shi-Yong Yu

Evidence for a rapid sea-level rise 7600 yr ago

Dating the transgression and subsequent regression in marginal basins of the southeastern Swedish Baltic Sea provides a new perspective of global ice-volume changes and the isostatic adjustment of the mantle after the retreat of the Scandinavian Ice Sheet from this area. Superimposed upon a smooth pattern of local sea-level rise, acceleration occurred ca. 7600 calibrated (cal) yr B.P., evidenced as a nearly synchronous flooding in six elevated basins ranging from 3.0 to 7.2 m above present sea level. We ascribe this rapid local sea-level rise of similar to 4.5 m to a sudden increase in ocean mass, most likely caused by the final decay of the Labrador sector of the Laurentide Ice Sheet. The subsequent monotonic fall of local sea level from ca. 6500 cal yr B.P. to the present is mainly an expression of the slow isostatic adjustment of the mantle. (Less)

Freshwater Outburst from Lake Superior as a Trigger for the Cold Event 9300 Years Ago

Down the Drain A pervasive cooling event affected much of the Northern Hemisphere approximately 9300 years ago. This event was accompanied by changes in ocean circulation in the North Atlantic, forced presumably by a large injection of fresh water produced by melting of the Laurentide Ice Sheet, but the source, magnitude, and routing of the meltwater remain unknown. Yu et al. (p. 1262, published online 29 April) present evidence that the trigger for this cooling episode was an outburst flood from Lake Superior. Reconstructing lake-level changes in the Superior basin suggests that a rapid fall of lake level of about 45 meters occurred 9300 years ago, possibly due to the sudden failure of a drift dam. Rapid drainage through the North Bay–Ottawa River–St. Lawrence River valleys into the North Atlantic should have been sufficient to disturb ocean circulation in line with the geologic record. The trigger for the dramatic North Atlantic cooling event 9300 years ago was an outburst flood from Lake Superior. Paleoclimate proxy records reveal a pervasive cooling event with a Northern Hemispheric extent ~9300 years ago. Coeval changes in the oceanic circulation of the North Atlantic imply freshwater forcing. However, the source, magnitude, and routing of meltwater have remained unknown. Located in central North America, Lake Superior is a key site for regulating the outflow of glacial meltwater to the oceans. Here, we show evidence for an ~45-meter rapid lake-level fall in this basin, centered on 9300 calibrated years before the present, due to the failure of a glacial drift dam on the southeast corner of the lake. We ascribe the widespread climate anomaly ~9300 years ago to this freshwater outburst delivered to the North Atlantic Ocean through the Lake Huron–North Bay–Ottawa River–St. Lawrence River valleys.

Holocene palaeoecology and shoreline displacement on the Biskopsmala Peninsula, southeastern Sweden

High-resolution palaeoecological proxies of pollen, macrofossils and diatoms from an isolation lake provide a long-term record of the Holocene landscape history and shoreline displacement on the Biskopsmala Peninsula in central Blekinge, SE Sweden. During the Preboreal/Boreal transition, the peninsula was sparsely vegetated by woodlands, along with lateglacial dwarf shrub/steppe communities. The lake basin was isolated from the shallow Yoldia Sea during this time. The regional climate improved from 10 700 cal. BP, evident as progressive expansion of Pinus -dominated mixed forest with deciduous trees. The lake basin was probably connected with the Ancylus Lake during the period 10 700-10 100 cal. BP. Subsequently the basin became isolated again, corresponding to the Early Littorina Sea phase. Replacement of freshwater diatoms by those with brackish-water affinity at 8100 cal. BP indicates the initial transgression of the Littorina Sea in this basin. But not until 7500 cal. BP were brackish conditions fully established. Peaks of brackish-marine diatoms and dinoflagellates during 7500-7000 cal. BP indicate increased saltwater inflow to the Baltic Sea in response to global meltwater pulse 3. However, interactive changes in seagrass and stonewort macrofossil concentrations suggest that three minor transgressions during 5900-5300, 5000-4700 and 4400-4000 cal. BP occurred locally, associated with centennial-scale variations in regional wind pattern or coastal storminess. By 3000 cal. BP, the lake basin was finally isolated from the Baltic, and thereafter the landscape on the peninsula became gradually more influenced by human activities.

New insights on the Yoldia Sea low stand in the Blekinge archipelago, southern Baltic Sea

Abstract One sediment core from the Järnavik bay in Blekinge archipelago has been investigated for its content of pollen and diatoms and its chemical properties. Two levels were also dated by radiocarbon. Based on the results the sediment sequence analysed has been divided into three environmental units largely corresponding to the lithology of the sequence. A lowermost unit consisting of weakly varved and homogeneous clay was deposited during the end of the brackish phase of the Yoldia Sea at a moderate water depth. On top of this unit a gyttja-clay unit was deposited. The onset of the deposition of this unit has been dated to c. 11 100 cal. yrs. BP. An increasing organic production and increased terrestrial influence is recorded in the chemical data and a very shallow water depth is indicated in the pollen and diatom flora. These results point to conditions in a bay probably isolated from the Yoldia Sea. A local tentative shore displacement curve have been constructed and it is proposed that this unit represents the low stand at c. -18 m during the Yoldia Sea stage in this part of the Baltic Sea basin. The uppermost unit consists of homogeneous clay with a low content of organic carbon. An increasing water depth is indicated by the composition of both pollen and diatoms. The diatom flora also displays an increase in freshwater species. This environmental change was probably the result of a transgression in the beginning of the Ancylus Lake stage.

Mid-Holocene Baltic Sea transgression along the coast of Blekinge, SE Sweden ancient lagoons correlated with beach ridges

Abstract The mid-Holocene Littorina transgression in southern Scandinavia is well documented. Multiple-stratigraphic sequences in ancient Littorina lagoons in the coastal area of Blekinge, SE Sweden reveal a maximum relative sea level of 7–8 m above present sea level between 8000–6000 cal. BP. Evidence for at least two transgression waves is found within this period. In this study these are documented in one modern lake and correlated with an ancient beach-lagoon stratigraphy. Furthermore, two younger transgressions are documented at one site, altogether establishing a firm transgression chronology for the time span 8000–4000 cal. BP (sea level 5–8 m a.s.l.) as a basis for understanding the dynamics of Baltic sea-level changes. Neolithic cultural layers are correlated to regression periods, indicating more favorable conditions for beach settlement between stormy transgression periods.

BEMMA: A Hierarchical Bayesian End-Member Modeling Analysis of Sediment Grain-Size Distributions

Sediment grain-size distributions provide rich information about sedimentary dynamics and potentially about environmental and climatic changes. However, entrainment, transport, and deposition, as a sequence of sorting process, modify original grain-size distributions of source materials, thereby resulting in complex distribution forms that are commonly multimodal and asymmetrical. However, neither traditional descriptive statistics nor curving fitting methods are able to address this complexity fully. End-member modeling analysis, essentially based on polytope expansion, stands out as a flexible and robust method for the unmixing of sediment grain-size distributions. Yet there are still several key issues that remain unresolved. Here a hierarchical Bayesian end-member modeling analysis of grain-size distributions, fully subject to the non-negative and unit-sum constraints on the distributions, is presented. Within the Bayesian framework, the number of end members, as well as the end-member spectra and fractions can be inferred sequentially using a reversible-jump Markov chain Monte Carlo algorithm in conjunction with Gibbs samplers. Test runs using both a synthetic and a real-world dataset from a small playa located on the southern margin of the Badain Jaran Desert, NW China, reveal that this model can yield geologically meaningful and mathematically optimal outputs, thereby providing a necessary complement and powerful alternative to the existing deterministic methods.

Holocene organic carbon burial rates in the southeastern Swedish Baltic Sea

To estimate Holocene changes in organic carbon mass accumulation rates (Corg MARs) in the southeastern Swedish Baltic Sea, two sediment cores were studied in combination with biogeochemical modelling. Prior to 11 300 cal. BP, Corg MARs were extremely low, indicating low organic matter production on the catchment of the Baltic Ice Lake. Following a brief regression, the Ancylus Lake stage occurred between 11 100 and 9800 cal. BP. Corg MARs increased substantially during this period because of enhanced washing in of terrestrial organic matter, when boreal forests were initially established. The prominent marine stage, known as the Littorina transgression between 8500 and 3000 cal. BP, is marked by a minor increase in Corg MARs. Our modelling reveals a changing terrestrial organic carbon input between 100 and 1000 g/m2 per yr that accounts for 30—80% of total organic carbon in sediments of the southeastern Swedish Baltic Sea, with maximum values (c. 20 × 103g/m2 per yr) occurring during the Bronze/Iron Age transition at about 3000 cal. BP. Corg MAR in the entire Baltic basin is estimated at 3.01 × 106 t/yr during the pre-industrial Holocene, comparable with other large inland water bodies. Regardless of the source of carbon, our data indicate that the Baltic basin is an important sedimentary reservoir for organic carbon storage and thus should be included in accounting for global terrestrial carbon cycling during the pre-industrial Holocene.