Markus Kienast

Termination II (T-II) has been considered a relatively smooth glacial–interglacial transition, punctuated by a single cold phase, Heinrich Stadial 11 (HS-11; ~136–129 ka), during which enhanced freshwater input from Northern Hemisphere ice sheets weakened the Atlantic Meridional Overturning Circulation (AMOC), thereby promoting a southward displacement of the Intertropical Convergence Zone (ITCZ). Here, we present oceanographic and runoff-sensitive proxy records from the Tobago Basin, north of the Orinoco Delta in the tropical Atlantic, that track meridional ITCZ migration during T-II. Consistent with expected effects of a southward ITCZ shift during HS-11, our records indicate reduced runoff and a deepened mixed layer. However, at ~134±1.4 ka, this pattern was reversed for several centuries, with increased runoff and a shallower mixed layer, suggesting a northward ITCZ shift that is at odds with the persistently southward position expected under a continuously weakened AMOC. This hydroclimate reversal coincided with North Atlantic sea-surface warming and reduced freshwater input from Northern Hemisphere ice sheets in other records, consistent with a centennial-scale reinvigoration of the AMOC during HS-11. Our findings add to recent evidence challenging the conventional view of HS-11 as an event with major AMOC weakening for ~7 kyr, and instead imply a more complex sequence of climate variability, with implications for understanding the evolution and dynamics of major deglaciations in general.

The Labrador Coastal Current (LCC) is a buoyancy driven current comprised of outflows from Baffin Bay and Hudson Bay that transports freshwater and nutrients to the Scotian Shelf and Gulf of Maine in the NW Atlantic. This study reconstructs Holocene flow speeds over the Labrador Shelf, using the sortable silt index at two core sites off northern and central Labrador. Reconstructions of current speeds at these two sites are similar through time and suggest that the LCC is controlled by Hudson Bay outflow, and experiences a 1,500 cyclicity consistent with the Arctic Oscillation and flow patterns from the NE Atlantic. Surface sediments suggest an increase in LCC velocity in recent decades, consistent with increased freshwater export from the Arctic, and possible recent weakening of the Atlantic meridional overturning circulation. Our new flow speed reconstructions differ from other major North Atlantic Holocene current speed records, but correspond to changes in salinity and bottom water temperature on the Labrador Shelf. This likely reflects the influence of cold and fresh Arctic waters versus warm, saline Atlantic waters. Results indicate recirculating Atlantic waters were likely the main LCC driver during an establishment period from 9 to 6 ka B.P whereas Hudson Bay outflow became the dominant control 6 ka B.P.—present, during a stabilization period once postglacial conditions were established.

The low-latitude flow of water masses from the Pacific to the Indian Ocean, the Indonesian Throughflow (ITF), is a choke point of the surface ocean return flow of the ocean conveyor belt. Even though the significance of the ITF for the modern global ocean circulation and climate has long been established, little is known about the hemispheric origin of the water masses contributing to its overall transport in the past. Here, we take advantage of the distinctly different isotopic composition of subsurface nitrate in the Northern and Southern Hemisphere source waters to document the admixture of these waters in the ITF through time. Our record of bulk sedimentary δ ¹⁵ N from the Banda Sea, at the heart of the ITF, shows that Southern Hemisphere-sourced subsurface waters contributed significantly to the total ITF transport during the last 800,000 years. Because Southern Ocean processes ultimately set the biogeochemical source signature of the Southern Hemisphere endmember, the Banda Sea record implies an important conduit by which high southern latitude climate and ocean variability is transmitted into the global ocean.

Low-strength sediment layers within continental slope strata precondition submarine sediment for failure, potentially leading to destructive tsunamis. Using geophysical and Ocean Drilling Program well data, here we show that the glide planes of widespread submarine failures in the northern South China Sea, dated to the glacial stages following the Mid-Pleistocene Transition, have higher opal content, particle size, and porosity, which reduce the undrained shear strength. Cyclic weak-layer deposition, modulated at Milankovitch time scale, was controlled by increased ocean primary productivity and sedimentation rates linked to high-amplitude sea-level fluctuations and intensified winter monsoons. This study represents an important step forward for understanding how climate influences the formation of weak layers and the stability of continental slope globally.

The Transpolar Drift (TPD) plays a crucial role in regulating Arctic climate and ecosystems by transporting fresh water and key substances, such as terrestrial nutrients and pollutants, from the Siberian Shelf across the Arctic Ocean to the North Atlantic. However, year-round observations of the TPD remain scarce, creating significant knowledge gaps regarding the influence of sea ice drift and ocean surface circulation on the transport pathways of Siberian fresh water and associated matter. Using geochemical provenance tracer data collected over a complete seasonal cycle, our study reveals substantial spatiotemporal variability in the dispersal pathways of Siberian matter along the TPD. This variability reflects dynamic shifts in contributions of individual Siberian rivers as they integrate into a large-scale current system, followed by their rapid and extensive redistribution through a combination of seasonal ice–ocean exchanges and divergent ice drift. These findings emphasize the complexity of Arctic ice–ocean transport pathways and highlight the challenges of forecasting their dynamics in light of anticipated changes in sea ice extent, river discharge, and surface circulation patterns.

The Northwest Atlantic continental shelf is a highly productive marine region with major uncertainties regarding the supply mechanisms of nutrients fueling productivity. This study uses nitrate isotopes (δ¹⁵N_NO3 and δ¹⁸O_NO3) from the Atlantic Zone Monitoring Program 2020 fall mission and an Optimum Multiparameter Analysis to evaluate on‐shelf nutrient transport versus biological supply pathways across three transects from the Laurentian Channel to the central Scotian Shelf. Surface waters showed an imprint of remineralized production, with partial nitrification creating an isotopically light (δ¹⁵N_NO3 ≥2.7‰) surface lens extending from Cabot Strait across the coastal Scotian Shelf. A concurrent enrichment in δ¹⁵N_NO3 and δ¹⁸O_NO3 (15N_NO3 signaling sedimentary denitrification in the deep Laurentian Channel. Conversely, subsurface nutrients on the Scotian Shelf were predominantly preformed (4.2‰)−characteristic of N₂ fixation in shallow NACW−indicate a stronger NACW imprint on the central Scotian Shelf compared to the Laurentian Channel and eastern shelf. Our results confirm the importance of slope water advection in supplying subsurface nutrients to the shelf while highlighting the seasonal imprint of recycled production on near‐shore surface waters during fall.

Several paleoceanographic and climate modeling studies have shown both oceanic and atmospheric teleconnections between climate in the tropics and the high latitudes on timescales ranging from decadal to multi-centennial. The last glacial period is characterized by millennial-scale abrupt warmings (interstadials) followed by rather gradual coolings to colder (stadials) Dansgaard-Oeschger (DO) events. The more pronounced of these stadial phases coincide with occurrences of ice-rafted debris in sediments from the mid-latitude Atlantic Ocean, referred to as Heinrich events (HE). Climate oscillations associated with DOs and HEs are also recorded in tropical climate archives around the Indian Ocean and on the Asian continent. However, forcing and response mechanisms of the Indo-Asian monsoon system and ocean-atmosphere exchange processes in conjunction with these millennial-scale oscillations are still poorly understood. Here, we present high-resolution geochemical and micropaleontological data from a sediment core located at 571 m water depth offshore Pakistan, representing the past 80,000 years at millennial-scale resolution.Alkenone unsaturation-derived sea surface temperature (SST) estimates show overall variations between 23 and 28°C. Millennial scale SST changes of 2°C are modulated by longer-term SST fluctuations. Interstadial intervals are characterized by higher organic carbon (TOC) concentrations, whereas sediments with low TOC contents mark stadials. Productivity-related and anoxia-indicating proxies show abrupt shifts with a 50-60 year duration at climate transitions, such as interstadial inceptions. Inorganic data consistently indicate that enhanced fluxes of terrestrial-derived sediments are paralleled by productivity maxima, and are characterized by an increased fluvial contribution from the Indus River during interstadials. The hydrogen isotopic composition of terrigenous plant waxes indicates that stadials are dry phases whereas humid conditions seem to have prevailed during interstadials. Stadials are characterized by an increased contribution of aeolian dust. HEs are especially dry, indicating a dramatically weakened Indian summer monsoon and increased continental aridity. The stable oxygen isotope (δ18O) records of the surface-dwelling foraminifera G. ruber and of the thermocline-dwelling P. obliquiloculata both show a strong correspondence to Greenland ice core δ18O, whereas the δ18O signal of benthic foraminifera (U. peregrina and G. affinis) reflects patterns similar to those observed in Antarctic ice core records. Distinct shifts in benthic δ18O during stadials indicate frequent injections of oxygen-rich intermediate water masses of Southern Ocean origin into the Arabian Sea. The most pronounced oceanographic changes occur during the transition and the termination of HE 4, respectively. Mg/Ca ratios of G. affinis show a rapid increase (decrease) of bottom water temperatures during the onset (termination) of HE 4, which is in good agreement to modelling studies. The hydrogen isotopic composition of terrigenous plant waxes indicates that HE 4 is much drier than the surrounding DOs.Overall, our results strengthen the notion that North Atlantic temperature changes and shifts in the hydrological cycle of the Indian monsoon system are closely coupled, with significant impacts on regional environmental conditions such as river discharge and ocean margin anoxia. These shifts were modulated by changes in the supply of water masses from the Southern Hemisphere.

In recent years, seagrass has been presented as a solution to sequester excess carbon emissions from the atmosphere, with studies reporting that seagrass meadows are responsible for burying as much as 10% of anthropogenic carbon per year (Fourqurean et al. 2012). However, this estimate has started to seem improbable as more recent research, specifically from North American study sites, are reporting carbon stock estimates much lower than the global average. Here, we present estimates of organic carbon (OC) stock in an eelgrass meadow on the Eastern Shore of Nova Scotia, Canada. To quantify sediment OC stock, we combined sediment geochemical analysis with geospatial mapping based on high-resolution optical aerial imagery collected by drone flights. Three sediment cores, plus a control, were extracted from the meadow in regions with differing levels of vegetative cover. The control core was used to establish a background signal for sediment OC, which we assume to be representative of nearshore unvegetated sediments in the region.Despite the health and anecdotally reported longevity of this eelgrass meadow measuring 4.7 Ha in size, the carbon stock is estimated to be less than 10 Mg OC/Ha. This is significantly lower than the global average estimates of ~163.3-660 Mg/Ha (Fourqurean et al. 2012) but is comparable to other reports emerging from the North American east coast (eg. 3.7 Mg/Ha from coastal Virginia, US: Greiner et al. 2013). From the individual core slices, the maximum sediment OC did not exceed 2.5 weight % even in the densest, healthiest part of the meadow. There was also a notable correlation between presence of coarse biomass and higher sediment OC in the bulk sample, suggesting that the carbon is mostly associated with living biomass rather than being buried and stored in sediments. Further, radiocarbon ages of the bulk OC of up to 1140 years in the topmost sediment layer imply a significant admixture of pre-aged, likely terrestrial, OC to the bulk OC, rendering the stock estimates absolute maximum estimates. Overall, this study adds to the growing body of evidence that suggests that global estimates of OC storage in eelgrass beds need to be carefully reevaluated.

The rapid increase of atmospheric carbon dioxide (CO2) and the concurrent decline in Δ14C during the last deglaciation were mainly ascribed to the release of old, 14C-depleted CO2 from an abyssal ocean reservoir, specifically the Southern Ocean, or the deep Pacific Ocean via intermediate waters. In support of this hypothesis, several records from intermediate waters around the globe depict a drop in Δ14C during the deglaciation. However, other records closer to the source regions of intermediate waters do not depict this anomaly and thus, question the hypothesis. Alternative scenarios include the release of 14C-depleted CO2 by hydrothermal vents, volcanoes and pockmarks. An ideal region to test the hypothesized scenarios is the western equatorial Pacific Ocean (WEP), where intermediate waters of southern and northern origin converge.We present paired planktic and benthic foraminiferal 14C ages from a depth transect (404 – 2210 m) of seven gravity cores from the WEP that cover the past 25 kyrs. Our records do not show any discernible Δ14C anomaly during the Last Glacial Maximum and initial deglaciation making the WEP an unlikely candidate for the release and ventilation of oceanic CO2 to the atmosphere. However, the intermediate-depth records consistently show anomalously low benthic Δ14C values during the final stage of the deglaciation and early Holocene. This Δ14C variability will be discussed in the context of potential sources and mechanisms.