Julia Gottschalk

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

The Mid-Pleistocene Transition (MPT), from ~ 1.2 to 0.6 million years is characterized by glacial interglacial cycles periodicity shifting from 41 kyr to ~100 kyr with larger amplitude variations (Pisias et Moore, 81; Imbrie et al., 93). This transition cannot be explained as a direct consequence of the astronomical forcing. Among the different mechanisms that has been suggested to explained this change in periodicity, a number involve a long-term decrease in atmospheric carbon, linked to greater carbon sequestration in the ocean. Various processes may have caused this carbon sequestration in the ocean (Paillard, 207; Willeit et al., 2019), including ocean temperature decrease, increased productivity in the Southern Ocean (Martinez-Garcia et al., 2011), and changes in deep ocean circulation (Raymo et al., 1997, Peña et Goldstein, 2012, Hasenfratz et al., 2019). For some processes, reconstructions indicate different timings, with mean surface temperature decrease occurring before 1.2 Myr (Snyder 2016) while ocean circulation changes started after 0.95 Myr (Raymo et al., 1997, Peña et Goldstein, 2012, Hasenfratz et al., 2019). Here we present the evolution of CO32- concentration along the MPT, in the Pacific sector of the Southern Ocean, that is linked to carbon accumulation. This record has been reconstructed from benthic foraminifera B/Ca ratio (Yu and Elderfield, 2007) from 1.25 to 0.45 Myr. While the two records from the Atlantic Ocean, North (Sosdian et al. 2018) and South (Farmer et al., 2019), indicate a reduction in CO32-, thus an increase in carbon accumulation, starting with the deep Atlantic Ocean circulation change at ~0.95 Myr (Raymo et al., 1997, Peña et Goldstein, 2012), these new South Pacific data indicate a delayed decrease by ~100kyr in agreement with a Pacific tropical record (Qin et al., 2022) obtained from changes in normalized weights of planktic foraminifera. This different timing, even within the Southern Ocean, questions the role of the balance between the dissolution/accumulation of carbonates between the Atlantic and Pacific basins. Hasenfratz et al.,2019, Science, 363, 1080–1084.Imbrie J. et al., 1993, Paleoceanography, https://doi.org/10.1029/93PA02751Paillard, D., 2017, Clim. Past, 13, 1259-1267.Peña L.D & Goldstein S.L., 2014, Science 345 (6194):318-22.Pisias, N.G. and T.C. Moore, 1981, EPSL, 52, 450-458, https://doi.org/10.1016/0012-821X(81)90197-7Qin, B. et al. (2022). Geophysical Research Letters, 49, e2021GL097121. https://doi. org/10.1029/2021GL097121Raymo, M.E. et al., 1997, Paleoceanography 12, 546–559. 15. 16.Snyder C.W., 2016, Nature, 538,226-228Sosdian, S. et al., (2018), Paleoceanography and Paleoclimatology, 33, 546–562., https://doi.org/10.1029/2017PA003312Willeit et al., 2019, Sci. Adv. 5 : eaav7337.Yu, J., & Elderfield, H. (2007), Earth and Planetary Science Letters, 258(1–2), 73–86. https://doi.org/10.1016/j.epsl.2007.03.025

Antarctic ice core evidence indicates that atmospheric CO2 levels increased during Heinrich Stadial (HS) 1 and the Younger Dryas (YD) during the last deglaciation. A substantial fraction of this carbon is believed to have stemmed from the ocean interior, released, in part, through enhanced wind-driven upwelling and air-sea CO2 exchange in the Southern Ocean. This was highlighted by two deglacial opal flux peaks identified in sediment core TN057-13-PC4 (53.17 °S, 5.13 °E, 2818 m water depth) from the Atlantic Southern Ocean south of the Polar Front, proximal to the Antarctic Divergence Zone (Anderson et al., 2009). However, there is limited information on changes in deep-ocean 14C ventilation and surface ocean hydrography in the Atlantic Antarctic Divergence, and their role in atmospheric CO2 variations during these two periods of deglacial CO2 rise. Here, we provide a new set of 12 mixed-benthic and 63 planktonic foraminiferal (i.e., Neogloboquadrina pachyderma) 14C ages obtained with a MIni-CArbon-DAting-System (MICADAS) in sediment core TN057-13-PC4, along with high-resolution multi-proxy (sub-)sea surface temperature reconstructions for the same site (N. pachyderma Mg/Ca ratios, TEX86, diatom assemblages). Our data help better constrain the nature, timing, and impacts of deep-ocean upwelling on surface ocean hydrography and on atmospheric CO2 exchange near the Antarctic Divergence of the Southern Ocean. Our data show strong (sub-)surface warming in the Antarctic Divergence during HS1 and YD that is accompanied by a rapid decline in benthic-minus-planktic 14C ages towards mean Holocene values at the onset of the deglaciation. We also observe millennial-scale increases in seawater d18O (paired N. pachyderma Mg/Ca-d18O analyses), hence local surface salinity and marked variations in 14C surface ocean reservoir ages that parallel changes in Antarctic sea ice extent. This corroborates previous evidence indicating increased upwelling of Circumpolar Deep Water in the Atlantic Antarctic Divergence during HS1 and YD, yet suggests an onset of strong Southern Ocean ventilation earlier than what is expected from increases in opal fluxes alone. Our data support a fundamental role of upwelling and CO2 outgassing in the Antarctic Divergence of the Southern Ocean in the two-step atmospheric CO2 rise during the last deglaciation, and further suggest that possible variations in CO2 solubility and sea-ice retreat amplified the effects of physical circulation changes on Southern Ocean air-sea CO2 exchange.References: Anderson, R.F., Ali, S., Bradtmiller, L.I., Nielsen, S.H.H., Fleisher, M.Q., Anderson, B., Burckle, L.H., 2009. Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO2. Science 323, 1443–1448. doi: 10.1126/science.116744

The mid-Pleistocene transition [MPT; between ~1.25 and 0.7 million years ago (Ma)] marks when Earth’s glacial pacing switched from 41,000- to 100,000-year cyclicity. Here, we present a South Pacific hydrogenous neodymium isotope record tracking changes in deep ocean circulation spanning 1.5 to 0.5 Ma. This record indicates that the MPT was not triggered by Northern Hemisphere (NH) processes. Instead, the growth of Antarctic ice sheets to a critical size (~1.8 Ma) primed the climate system for the MPT, with iceberg release and melting transferring buoyancy out of the Southern Ocean. This iceberg-associated buoyancy redistribution ultimately reduced deep water formation in the NH, an observation supported by our data. Once the MPT was initiated in the Southern Hemisphere, its subsequent climatic changes were dominated by NH processes.

Marine diseases threaten ecosystems and aquaculture, additionally harming endangered species including the European flat oyster (Ostrea edulis ). Although international restoration efforts follow biosecurity protocols, ocean currents may transport pathogens to restoration sites. Here, we used biophysical Lagrangian dispersal simulations to map upper-bound exposure risk of the oyster pathogen Bonamia ostreae across the North-West European shelf. We developed a workflow with pre-aggregated connectivity components allowing analyses of multiple scenarios without rerunning simulations. We found typical dispersal distances from 30 km for free pathogens to 50-60 km for infected larvae, with high spatial heterogeneity emphasizing the need for site-specific simulations. About 30% of restoration sites showed a continuously high potential exposure to pathogen arrival. We identified highly connected diseased sites that may drive transmission. We provide a scaling factor to calculate pathogen exposure at target locations and a tool for selecting low-risk sites. This transferable workflow supports site-specific spatial planning in restoration, aquaculture, and disease monitoring.

The climatic optimum of Marine Isotope Stage (MIS) 11, MIS 11c, was a remarkably long warmer‐than‐present climate interval with prolonged elevated atmospheric CO ₂ levels. While the Pacific Southern Ocean has a strong leverage on atmospheric CO ₂ through oceanic CO ₂ release, past changes in carbon cycle dynamics in this region and their influence on atmospheric CO ₂ during MIS 11 and the glacial inception to MIS 10 remain poorly constrained. Here, we assess changes in vertical mixing (planktic vs. benthic foraminiferal stable carbon isotope gradients) and respired carbon storage ( Cibicides kullenbergi B/Ca) in the central Pacific Southern Ocean at International Ocean Discovery Program Site U1540 for MIS 11 and the onset of glacial MIS 10, complemented with previous reconstructions of deep ocean oxygenation (foraminiferal U/Mn), ocean temperature ( Uvigerina spp. Mg/Ca, Neogloboquadrina pachyderma abundances) and ocean circulation (foraminiferal/fish debris Nd isotopes). Our data reveal a temporal offset between ocean cooling/global ice sheet growth and Southern Ocean carbon storage changes after peak MIS 11c warmth. This coincides with minimum obliquity, which favors southern‐hemisphere westerly wind (SWW)‐driven upwelling of CO ₂ ‐rich waters in the Southern Ocean and CO ₂ evasion (via obliquity control on meridional pressure gradients and SWW position/strength), explaining high sustained atmospheric CO ₂ levels during MIS 11c. The glacial inception following peak MIS 11c thus shows opposing effects of low obliquity favoring ice sheet growth through polar cooling and poleward moisture supply, yet maintaining high atmospheric CO ₂ levels through continued Southern Ocean CO ₂ outgassing that likely decelerated global ice sheet growth.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.