Lester Lembke-Jene

Sea surface temperature variability and sea‐ice extent in the subarctic northwest Pacific during the past 15,000 years

Past changes in North Pacific sea surface temperatures and sea-ice conditions are proposed to play a crucial role in deglacial climate development and ocean circulation but are less well known than from the North Atlantic. Here, we present new alkenone-based sea surface temperature records from the subarctic northwest Pacific and its marginal seas (Bering Sea and Sea of Okhotsk) for the time interval of the last 15 kyr, indicating millennial-scale sea surface temperature fluctuations similar to short-term deglacial climate oscillations known from Greenland ice-core records. Past changes in sea-ice distribution are derived from relative percentage of specific diatom groups and qualitative assessment of the IP25 biomarker related to sea-ice diatoms. The deglacial variability in sea-ice extent matches the sea surface temperature fluctuations. These fluctuations suggest a linkage to deglacial variations in Atlantic meridional overturning circulation and a close atmospheric coupling between the North Pacific and North Atlantic. During the Holocene the subarctic North Pacific is marked by complex sea surface temperature trends, which do not support the hypothesis of a Holocene seesaw in temperature development between the North Atlantic and the North Pacific. Key Points: - Millennial-scale changes in SST in the North Pacific during the last 15 kyr - Changes in sea-ice extent suggest a close coupling to SST fluctuations - Middle to late Holocene SSTs show no clear SST trend in the North Pacific

Glacial reduction and millennial-scale variations in Drake Passage throughflow

Significance The Drake Passage (DP) represents the most important oceanic gateway along the pathway of the world’s largest current: the Antarctic Circumpolar Current (ACC). Resolving changes in the flow of circumpolar water masses through the DP is crucial for advancing our understanding of the Southern Ocean’s role in affecting ocean and climate change on a global scale. We reconstruct current intensity from marine sediment records around the southern tip of South America with unprecedented millennial-scale resolution covering the past ∼65,000 y. For the last glacial period, we infer intervals of strong weakening of the ACC entering the DP, implying an enhanced export of northern ACC surface and intermediate waters into the South Pacific Gyre and reduced Pacific–Atlantic exchange through the cold water route. The Drake Passage (DP) is the major geographic constriction for the Antarctic Circumpolar Current (ACC) and exerts a strong control on the exchange of physical, chemical, and biological properties between the Atlantic, Pacific, and Indian Ocean basins. Resolving changes in the flow of circumpolar water masses through this gateway is, therefore, crucial for advancing our understanding of the Southern Ocean’s role in global ocean and climate variability. Here, we reconstruct changes in DP throughflow dynamics over the past 65,000 y based on grain size and geochemical properties of sediment records from the southernmost continental margin of South America. Combined with published sediment records from the Scotia Sea, we argue for a considerable total reduction of DP transport and reveal an up to ∼40% decrease in flow speed along the northernmost ACC pathway entering the DP during glacial times. Superimposed on this long-term decrease are high-amplitude, millennial-scale variations, which parallel Southern Ocean and Antarctic temperature patterns. The glacial intervals of strong weakening of the ACC entering the DP imply an enhanced export of northern ACC surface and intermediate waters into the South Pacific Gyre and reduced Pacific–Atlantic exchange through the DP (“cold water route”). We conclude that changes in DP throughflow play a critical role for the global meridional overturning circulation and interbasin exchange in the Southern Ocean, most likely regulated by variations in the westerly wind field and changes in Antarctic sea ice extent.

Variations of the oxygen minimum zone of the Okhotsk Sea during the last 50 ka as indicated by benthic foraminiferal and biogeochemical data

Benthic foraminiferal and sediment biogeochemical data (total organic carbon, calcium carbonate and biogenic opal contents) in two cores (1265 and 1312 m water depths) from the southeastern Sakhalin slope and one core (839 m water depth) from the southwestern Kamchatka slope were investigated to reconstruct variations of the oxygen minimum zone during the last 50 ka in the Okhotsk Sea. The oxygen minimum zone was less pronounced during cooling in the MIS 2 that is suggested to be caused by a maximal expansion of sea ice cover, decrease of marine productivity and increase of production of the oxygenated Okhotsk Sea Intermediate Water (OSIW). A two-step-like strengthening of oxygen minimum zone during the warmings in the Termination 1a and 1b was linked to (1) enhanced oxygen consumption due to degradation of large amount of organic matter in the water column and bottom sediments, originated from increased marine productivity and supply of terrigenous material from the submerged northern shelves; (2) sea ice cover retreat and reduction of OSIW production; (3) freely inflow of the oxygen-depleted intermediate water mass from the North Pacific.

Evidence for enhanced convection of North Pacific Intermediate Water to the low‐latitude Pacific under glacial conditions

We provide high-resolution foraminiferal stable carbon isotope (δ13C) records from the subarctic Pacific and Eastern Equatorial Pacific (EEP) to investigate circulation dynamics between the extratropical and tropical North Pacific during the past 60 kyr. We measured the δ13C composition of the epibenthic foraminiferal species Cibicides lobatulus from a shallow sediment core recovered from the western Bering Sea (SO201-2-101KL; 58°52.52′N, 170°41.45′E; 630 m water depth) to reconstruct past ventilation changes close to the source region of Glacial North Pacific Intermediate Water (GNPIW). Information regarding glacial changes in the δ13C of subthermocline water masses in the EEP is derived from the deep-dwelling planktonic foraminifera Globorotaloides hexagonus at ODP Site 1240 (00°01.31′N, 82°27.76′W; 2921 m water depth). Apparent similarities in the long-term evolution of δ13C between GNPIW, intermediate waters in the eastern tropical North Pacific and subthermocline water masses in the EEP suggest the expansion of relatively 13C-depleted, nutrient-enriched, and northern sourced intermediate waters to the equatorial Pacific under glacial conditions. Further, it appears that additional influence of GNPIW to the tropical Pacific is consistent with changes in nutrient distribution and biological productivity in surface waters of the glacial EEP. Our findings highlight potential links between North Pacific mid-depth circulation changes, nutrient cycling, and biological productivity in the equatorial Pacific under glacial boundary conditions.

Pacific‐Atlantic Circumpolar Deep Water coupling during the last 500 ka

Investigating the interbasin deepwater exchange between the Pacific and Atlantic Oceans over glacial-interglacial climate cycles is important for understanding circum-Antarctic Southern Ocean circulation changes and their impact on the global Meridional Overturning Circulation. We use benthic foraminiferal δ13C records from the southern East Pacific Rise to characterize the δ13C composition of Circumpolar Deep Water in the South Pacific, prior to its transit through the Drake Passage into the South Atlantic. A comparison with published South Atlantic deepwater records from the northern Cape Basin suggests a continuous water mass exchange throughout the past 500 ka. Almost identical glacial-interglacial δ13C variations imply a common deepwater evolution in both basins suggesting persistent Circumpolar Deep Water exchange and homogenization. By contrast, deeper abyssal waters occupying the more southern Cape Basin and the southernmost South Atlantic have lower δ13C values during most, but not all, stadial periods. We conclude that these values represent the influence of a more southern water mass, perhaps Antarctic Bottom Water (AABW). During many interglacials and some glacial periods, the gradient between Circumpolar Deep Water and the deeper southern Cape Basin bottom water disappears suggesting either no presence of AABW or indistinguishable δ13C values of both water masses.

Rapid shift and millennial-scale variations in Holocene North Pacific Intermediate Water ventilation

Significance The North Pacific hosts extensive oxygen minimum zones. Ventilation of North Pacific Intermediate Water mitigates hypoxia in thermocline waters not under influence of ocean–atmosphere processes. Instrumental datasets show recent decadal decreases in O2, but millennial-scale natural variations in mesopelagic ventilation might be large and are not understood well. We reconstruct Holocene ventilation changes in a key region (Okhotsk Sea). Modern ventilation and O2 levels are a relatively recent feature. In the warmer-than-present Early Holocene, middepth O2 concentrations were 25 to 50% reduced, with significant millennial-scale variations. A sudden ventilation decrease six thousand years ago is linked to higher ocean temperatures, sea ice loss, and higher remineralization, corroborated by results from paleoclimate modeling, providing constraints for future warming scenarios. The Pacific hosts the largest oxygen minimum zones (OMZs) in the world ocean, which are thought to intensify and expand under future climate change, with significant consequences for marine ecosystems, biogeochemical cycles, and fisheries. At present, no deep ventilation occurs in the North Pacific due to a persistent halocline, but relatively better-oxygenated subsurface North Pacific Intermediate Water (NPIW) mitigates OMZ development in lower latitudes. Over the past decades, instrumental data show decreasing oxygenation in NPIW; however, long-term variations in middepth ventilation are potentially large, obscuring anthropogenic influences against millennial-scale natural background shifts. Here, we use paleoceanographic proxy evidence from the Okhotsk Sea, the foremost North Pacific ventilation region, to show that its modern oxygenated pattern is a relatively recent feature, with little to no ventilation before six thousand years ago, constituting an apparent Early–Middle Holocene (EMH) threshold or “tipping point.” Complementary paleomodeling results likewise indicate a warmer, saltier EMH NPIW, different from its modern conditions. During the EMH, the Okhotsk Sea switched from a modern oxygenation source to a sink, through a combination of sea ice loss, higher water temperatures, and remineralization rates, inhibiting ventilation. We estimate a strongly decreased EMH NPIW oxygenation of ∼30 to 50%, and increased middepth Pacific nutrient concentrations and carbon storage. Our results (i) imply that under past or future warmer-than-present conditions, oceanic biogeochemical feedback mechanisms may change or even switch direction, and (ii) provide constraints on the high-latitude North Pacific’s influence on mesopelagic ventilation dynamics, with consequences for large oceanic regions.

Deglacial mobilization of pre-aged terrestrial carbon from degrading permafrost

The mobilization of glacial permafrost carbon during the last glacial–interglacial transition has been suggested by indirect evidence to be an additional and significant source of greenhouse gases to the atmosphere, especially at times of rapid sea-level rise. Here we present the first direct evidence for the release of ancient carbon from degrading permafrost in East Asia during the last 17 kyrs, using biomarkers and radiocarbon dating of terrigenous material found in two sediment cores from the Okhotsk Sea. Upscaling our results to the whole Arctic shelf area, we show by carbon cycle simulations that deglacial permafrost-carbon release through sea-level rise likely contributed significantly to the changes in atmospheric CO2 around 14.6 and 11.5 kyrs BP.Permafrost-derived carbon (C) may have been an additional source of greenhouse gases during the last glacial-interglacial transition. Here the authors show that ancient C from degrading permafrost was mobilised during phases of rapid sea-level rise, partially explaining changes in atmospheric CO2 and ∆14C.

The european research icebreaker AURORA BOREALIS : conceptual design study - summary report = Der europäische Forschungseisbrecher AURORA BOREALIS : Konzeptioneller Schiffsentwurf - Abschlussbericht

SUMMARY Dedicated research vessels that are capable to operate during all seasons of the year and under unfavourable weather conditions in the central Arctic Ocean and in the Southern Ocean are required to fulfil the new needs of polar ocean research for all marine disciplines. Today, no available research vessel has these required capabilities. Thus, the initiative was taken to develop a novel and dedicated research icebreaker with technical capabilities hitherto unrealised, which will enable the vessel to autonomously operate in the Arctic Ocean even during the severest ice conditions in the deep winter serving all marine disciplines of polar research including deep-sea drilling: The AURORA BOREALIS. The technical conceptual design of this research icebreaker with scientific deepsea drilling capability AURORA BOREALIS is summarised in this report. AURORA BOREALIS is planned to be a „European scientific flagship facility‟ open to non-European partners, a multidisciplinary platform for studies ranging from the sub-seafloor into the atmosphere. The ability of AURORA BOREALIS to penetrate into the harshest conditions on Earth and to carry out research in the polar winter can set new standards in the fields of polar research and naval architecture, including environmental safety and sustainability of the highest standards. Currently, no polar research vessel has the capability to autonomously operate in pack ice outside the optimal ice conditions of the late summer season. AURORA BOREALIS, in contrast, is planned as a multi-purpose icebreaking research vessel for Arctic and Antarctic operations with the capability to autonomously navigate in sea-ice with a thickness of more than 2.5 metres. This will for the first time facilitate year-round research, e.g. on the nature of global environmental change. The ship shall have the unique capability to perform scientific deep-sea drilling operations in water depths between 100 and 5000 metres with a penetration of more than 1000 metres into the seafloor, even while being located amid drifting pack-ice fields. To perform these drilling operations AURORA BOREALIS has to be kept exactly on position. A dynamic positioning system capable for manoeuvring and staying on position in drifting sea-ice is mandatory for this task – an absolute novelty in the shipping industry. Another unique characteristic of AURORA BOREALIS are the two moon pools (7 x 7 meters each) in the midst of the hull that reach into the water below the vessel and enable scientists to deploy equipment into the ocean without being subject to wind, waves and ice. The aft moon pool is mainly dedicated to drilling operations, while the forward moon pool is reserved for most other scientific equipment. This allows as a first the deployment of very sensitive and expensive equipment, e.g. remotely operated (ROVʼs) or autonomous (AUVʼs) underwater vehicles within completely closed sea ice cover. Scientific laboratories are located on several decks around the moon pool, which is designed in an atrium-like shape with circular walkways and preparation areas. In order to optimally equip the ship even for mission specific expeditions, containerized laboratories can be also loaded here and become fully integrated into the scientific workflow on board.