ALBERT

Description: Due to its strong influence on heat and moisture exchange between the ocean and the atmosphere, sea ice is an essential component of the global climate system. In the context of its alarming decrease in terms of concentration, thickness and duration, understanding the processes controlling sea-ice variability and reconstructing paleo-sea-ice extent in polar regions have become of great interest for the scientific community. In this study, for the first time, IP25, a recently developed biomarker sea-ice proxy, was used for a high-resolution reconstruction of the sea-ice extent and its variability in the western North Pacific and western Bering Sea during the past 18,000 years. To identify mechanisms controlling the sea-ice variability, IP25 data were associated with published sea-surface temperature as well as diatom and biogenic opal data. The results indicate that a seasonal sea-ice cover existed during cold periods (Heinrich Stadial 1 and Younger Dryas), whereas during warmer intervals (Bolling-Allerod and Holocene) reduced sea ice or ice-free conditions prevailed in the study area. The variability in sea-ice extent seems to be linked to climate anomalies and sea-level changes controlling the oceanographic circulation between the subarctic Pacific and the Bering Sea, especially the Alaskan Stream injection though the Aleutian passes.

Description: TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms) is a sea surface temperature (SST) proxy based on the distribution of archaeal isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs). In this study, we appraise the applicability of TEX86 and View the MathML sourceTEX86L in subpolar and polar regions using surface sediments. We present TEX86 and View the MathML sourceTEX86L data from 160 surface sediment samples collected in the Arctic, the Southern Ocean and the North Pacific. Most of the SST estimates derived from both TEX86 and View the MathML sourceTEX86L are anomalously high in the Arctic, especially in the vicinity of Siberian river mouths and the sea ice margin, plausibly due to additional archaeal contributions linked to terrigenous input. We found unusual GDGT distributions at five sites in the North Pacific. High GDGT-0/crenarchaeol and GDGT-2/crenarchaeol ratios at these sites suggest a substantial contribution of methanogenic and/or methanotrophic archaea to the sedimentary GDGT pool here. Apart from these anomalous findings, TEX86 and View the MathML sourceTEX86L values in the surface sediments from the Southern Ocean and the North Pacific do usually vary with overlaying SSTs. In these regions, the sedimentary TEX86-SST relationship is similar to the global calibration, and the derived temperature estimates agree well with overlaying annual mean SSTs at the sites. However, there is a systematic offset between the regional View the MathML sourceTEX86L-SST relationships and the global calibration. At these sites, temperature estimates based on the global View the MathML sourceTEX86L calibration are closer to summer SSTs than annual mean SSTs. This finding suggests that in these subpolar settings a regional View the MathML sourceTEX86L calibration may be a more suitable equation for temperature reconstruction than the global calibration.

Description: Overall goal of our study of sediment material collected during RV Sonne Cruise 202 (INOPEX) in 2009 (Gersonde et al., Curise Report 2009), is the reconstruction of the short-term variability of sea-ice, sea-surface temperature (SST), primary productivity and terrigenous input in the subpolar North Pacific/Bering Sea and their relationship to global climate change, using organic-geochemical proxies (i.e. organic-geochemical bulk parameters and specific biomarkers such as: TOC, hydrogen indices; long-chain n-alkanes, sterols, alkenones; Uk37 and TEX86-Index; BIT-Index; HBIs, IP25, PIP25). In a first phase, these organic-geochemical proxies have been determined in surface sediments. The results show that the biomarker proxies reflect modern sea-ice and SST distributions as well as areas of increased primary productivity and increased input of terrigenous (organic) matter quite well. In a second phase of the project, the biomarkers have been determined in three selected sediment cores: Core SO202-18-6 (Umnak Plateau/Bering Sea; 60.127°N, 179.444°W; water depth 1105 m; core length 7.21 m; age interval 0 to 14 kyr.BP). Core SO202-07-6 (Detroit Seamount/western subpolar North Pacific; 51.272°N, 167.700°W; water depth 2340 m WD; core length 4.69 m; age interval MIS 1 to 3). Core SO202-27-6 (Patton Seamount/eastern subpolar North Pacific; 54.296°N, 149.600°W; water depth 2919 m; core length 2.91 m: age interval MIS 1 to 3). Here, we concentrate especially on the variability of sea-ice cover and sea-surface temperature, using the newly developed sea-ice proxy IP25 (Belt et al., 2007) and alkenone data, respectively, determined in the AMS14C-dated Core SO202-18-6. Based on these biomarker records, sea-ice cover and SST changed significantly in the northern Bering Sea during Deglacial-Holocene times. The Younger Dryas interval is characterized by extended sea-ice cover, coinciding with a drop in SST to 2-4°C. With the end of the Younger Dryas, between 460 and 420 cmbsf, sea-ice cover decreased with increasing SST. Between 420 and 120 cmbsf representing the early Holocene Thermal Maximum, IP25 is absent and maximum SST of about 6°C was reached. During the upper 120 cmbsf representing the late Holocene, IP25 occurred again and increased towards the top, paralleled by a decrease in SST of about 3°C. A very similar contemporaneous trend of increasing sea-ice cover during the late Holocene was recorded in the northern North Atlantic, paralleled by an advance of glaciers in Norway (Müller et al., 2009; 2012).

Description: Over the last three decades, the Bering Sea has undergone dramatic changes in its physical and biological environment due to anthropogenic warming, accompanied by a dramatic shift in sea-ice cover, thickness, and duration of the ice season. These recent observations have increased the interest of the paleo-community in reconstructing the past variability of sea-surface characteristics in this region. The main objectives of this study were the millennial-scale reconstruction of the variability of surface-water conditions and terrigenous input in the subarctic Pacific and Bering Sea over the last glacial-deglacial-Holocene time interval (25 ka) in order to investigate their relationship to global climate change. For this purpose, changes in organic geochemical composition in surface sediments and sediment cores, collected during the SO202- INOPEX RV Sonne and SO201-KALMAR R/V Sonne cruises were investigated. The main results of this thesis are presented in three manuscripts. In order to determine the applicability of specific biomarkers, in view of future paleoclimatologic and paleoceanographic reconstructions in the subarctic Pacific and the Bering Sea, the first manuscript was dedicated to the reconstruction of modern sea-surface characteristics, i.e., sea-surface temperature (SST) and sea-ice cover in this area. Based on hydrogen index values and the distribution of long-chain n-alkanes and specific sterols in surface sediment, we show that different organiccarbon sources prevailed in the study area. In the Bering Sea, organic matter has a predominantly marine origin, caused by high primary production, whereas in the North Pacific, organic carbon originates mostly from terrestrial higher plants, probably related to dust input from Asia. The results from the alkenone-based SST reconstruction demonstrate that the Sikes et al. (1997) calibration seems to be more accurate and matches the summer SSTs in the eastern North Pacific and the Bering Sea better than the Müller et al. (1998) calibration. In this study, we also show that the distribution of the novel sea-ice proxy IP25 in surface sediments mirror the modern spring sea-ice distribution and demonstrates the potential of this proxy to track past variations in sea-ice cover in the study area. In the second manuscript we use IP25 abundances, alkenone-based sea-surface temperatures, diatom and biogenic opal data from three sediment cores from the western North Pacific and western Bering Sea to reconstruct the variability of sea-ice extent during the past 18 ka. In general, there is a very good correlation between the biomarkers-based and the diatoms-based sea-ice records. The results demonstrate that a dominantly permanent sea-ice cover prevailed in the western Bering Sea during cold periods (Heinrich Stadial 1 and Younger Dryas), whereas reduced sea-ice or ice-free conditions existed during warmer intervals (Bølling/Allerød and Holocene). Warm intervals of reduced sea ice coincide with increased biogenic opal, indicating increased primary production. In the last manuscript, the millennial-scale of sea-ice reconstruction was extended to include the northeastern Bering Sea and the eastern and western subarctic Pacific. The results show that an extensive sea-ice cover prevailed over large parts of the subarctic Pacific and the Bering Sea during the LGM. The deglaciation-Holocene time interval is characterized by rapid sea-ice advance and retreat. During cold periods (Heinrich Stadial 1 and Younger Dryas) seasonal sea-ice cover generally coincides with low alkenone SSTs and low primary productivity. Conversely, during warmer intervals (Bølling/Allerød and Holocene) predominance of reduced sea ice or ice-free conditions are generally associated with increase in alkenone SSTs and primary productivity. However, in the northern Bering Sea continental shelf ice-free conditions prevailing during the Holocene Thermal Maximum shifted to marginal sea-ice conditions at the onset of the Mid Holocene. In summary, the work in this thesis demonstrates that sea-ice extent in the subarctic Pacific and the Bering Sea was highly variable during the last 25,000 years. The variability can be explained by a combination of local factors (e.g. solar insolation), as well as global climate anomalies (e.g. Bølling/Allerød and Younger Dryas) and sea-level changes controlling the oceanographic circulation between the subarctic Pacific and the Bering Sea.

Description: This study focusses on the last glacial–deglacial–Holocene spatial and temporal variability in sea-ice cover based on organic geochemical analyses of marine sediment cores from the subarctic Pacific and the Bering Sea. By means of the sea-ice proxy “IP25” and phytoplankton-derived biomarkers (specific sterols and alkenones), we reconstruct the spring sea-ice conditions, (summer) sea-surface temperature (SST) and primary productivity, respectively. The large variability of sea ice was explained by a combination of local and global factors, such as solar insolation, global climate anomalies and sea-level changes controlling the oceanographic circulation and water mass exchange between the subarctic Pacific and the Bering Sea. During the Last Glacial Maximum, extensive sea-ice cover prevailed over large part of the subarctic Pacific and the Bering Sea. The following deglaciation is characterized by a rapid sea-ice advance and retreat. During cold periods (Heinrich Stadial 1 and Younger Dryas) seasonal sea-ice cover generally coincided with low alkenone SSTs and low primary productivity. Conversely, during warmer intervals (Bølling/Allerød, Early Holocene) reduced sea-ice or ice-free conditions prevailed in the study area. At the northern Bering Sea continental shelf a late-Early/Mid Holocene shift to marginal sea-ice conditions is in line with the simultaneous wide-spread sea-ice recovery observed in the other Arctic marginal seas and is likely initiated by the lower Northern Hemisphere insolation and surface-water cooling.