Disentangling the effects of particles and circulation on 231Pa/230Th during Heinrich Stadials

It has been shown that during Heinrich stadials northern deep water production ceased leading to an enhanced inflow of southern sourced water. Although Heinrich events are not considered to represent the primary trigger of Heinrich stadials the reorganisation of Atlantic ocean dynamics during their occurrences is an active field of research. In particular, Heinrich stadial 2 (HS2) is of high interest, based on the observation that the interplay with the climate system was very different during HS2 compared to HS1, although the magnitude of iceberg and freshwater discharge was similar (Hemming, 2004). During HS2 sea-level was still decreasing while the atmospheric CO­2 content was relatively stable unlike the climatic evolution during Heinrich HS1.The notion of a reduced Atlantic Meridional Overturning Circulation (AMOC) during Heinrich Stadials is mainly strengthened by the 231Pa/230Th records from the Bermuda Rise. However, other influencing factors, capable of increasing the sedimentary 231Pa/230Th without according decreases in AMOC strength, need to be considered as well. Besides biogenic opal, high dust fluxes may also result in enhanced scavenging rate of both radionuclides and consequently higher sedimentary 231Pa/230Th signals, since another distinct feature that accompanies Heinrich Stadials is the high atmospheric concentration of dust in the northern hemisphere. Furthermore, high dust concentrations might be an indicator of a vigorous wind system and therefore strong ocean mixing, which can lead to the enhanced formation of nepheloid layers These layers are suspected to cause strong bottom scavenging and consequently high sedimentary 231Pa/230Th. Very high dust fluxes were observed e.g. during HS2 and MIS4. Here, we compare 231Pa/230Th with dust records in order to disentangle the effects of scavenging and circulation on the recorded sedimentary 231Pa/230Th from the northwestern Atlanti

Arctic drainage of Laurentide Ice Sheet meltwater throughout the past 14,700 years

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Sufke, F., Gutjahr, M., Keigwin, L. D., Reilly, B., Giosan, L., & Lippold, J. Arctic drainage of Laurentide Ice Sheet meltwater throughout the past 14,700 years. Communications Earth & Environment, 3(1), (2022): 98, https://doi.org/10.1038/s43247-022-00428-3.During the last deglaciation substantial volumes of meltwater from the decaying Laurentide Ice Sheet were supplied to the Arctic, Gulf of Mexico and North Atlantic along different drainage routes, sometimes as catastrophic flood events. These events are suggested to have impacted global climate, for example initiating the Younger Dryas cold period. Here we analyze the authigenic Pb isotopic composition of sediments in front of the Arctic Mackenzie Delta, a sensitive tracer for elevated freshwater runoff of the retreating Laurentide Ice Sheet. Our data reveal continuous meltwater supply to the Arctic along the Mackenzie River since the onset of the Bølling–Allerød. The strongest Lake Agassiz outflow event is observed at the end of the Bølling–Allerød close to the onset of the Younger Dryas. In context of deglacial North American runoff records from the southern and eastern outlets, our findings provide a detailed reconstruction of the deglacial drainage chronology of the disintegrating Laurentide Ice Sheet.Open Access funding enabled and organized by Projekt DEAL

Changes of the Atlantic meridional overturning circulation of the past 30ka recorded in a depth transect at the Blake Outer Ridge

Oceans and climate are a tightly coupled system interacting with each other in various ways such as storage of carbon dioxide in the deep ocean. Within the global conveyor belt the Atlantic Meridional Overturning Circulation (AMOC) holds a key function, transporting warm salty surface waters from the tropical to the northern Atlantic where deep water formation takes place. Following the continental rise of North America this newly formed deep water propagates southward as Western Boundary Undercurrent (WBUC) ventilating the deep Atlantic. In the past (e.g. the last glacial cycle) strength and geometry of the AMOC have changed significantly. This study aims to provide a better understanding of the temporal and spatial (also depth depended) evolution of the AMOC in the western Atlantic sector since the last glacial (∼30 ka). We have investigated four sediment cores of the Blake Outer Ridge (30°N, 74°W; ODP 1059 to 1062) in a depth transect from 3000 to 4700 m water depth in the main flow path of the WBUC. We measured four down-core profiles of neodymium (εNd) and 231Pa/230Th isotopes for the reconstruction of water mass provenance and circulation strength of the last ∼30 ka. In contrast to published Nd isotope and 231Pa/230Th records from the Blake Ridge area our records are of unprecedented resolution, resolving climate key features of the North Atlantic region: Heinrich Stadials (HS) 1 and 2, the Last Glacial Maximum (LGM), the Bølling-Allerød and Younger Dryas (YD). Radiogenic Nd isotope signatures during the LGM reveal AABW to be the prevalent water mass in the deep western North Atlantic. The trend to more unradiogenic signatures during the deglaciation point to an increased formation of NADW which was again replaced by AABW during YD. The Holocene shows the most unradiogenic signatures and therefore established NADW. The circulation strength-proxy 231Pa/230Th indicates reduced LGM deep circulation, a pronounced slowdown during HS1 and a strong and deep circulation during the Holocene. Compared to isotopic records from the Bermuda Rise (ODP 1063) we found depth depended geometry changes of the WBUC which have occurred through the last glacial. Here, we focus on how deep northern sourced water has reached during phases of reduced circulation (indicated by increased 231Pa/230Th ratios) and the timing of this southward progradation of lower NADW

Deglacial patterns of South Pacific overturning inferred from 231Pa and 230Th

The millennial‐scale variability of the Atlantic Meridional Overturning Circulation (AMOC) is well documented for the last glacial termination and beyond. Despite its importance for the climate system, the evolution of the South Pacific overturning circulation (SPOC) is by far less well understood. A recently published study highlights the potential applicability of the 231Pa/230Th‐proxy in the Pacific. Here, we present five sedimentary down‐core profiles of 231Pa/230Th‐ratios measured on a depth transect from the Pacific sector of the Southern Ocean to test this hypothesis using downcore records. Our data are consistent with an increase in SPOC as early as 20 ka that peaked during Heinrich Stadial 1. The timing indicates that the SPOC did not simply react to AMOC changes via the bipolar seesaw but were triggered via Southern Hemisphere processes

Constraints on the northwestern Atlantic deep water circulation from 231 Pa/ 230 Th during the last 30,000 years

Global climatic changes during the last Glacial and Deglacial have been related to variations of the Atlantic Meridional Overturning Circulation (AMOC). Here, we present new and refined 231Pa/230Th down‐core profiles extending back to 30 ka BP from the northwestern Atlantic along the Atlantic Deep Western Boundary Current (DWBC), which is the main component of the southward deep backflow of the AMOC. Besides the well‐known Bermuda Rise records, available high‐resolution 231Pa/230Th data in the northwestern Atlantic are still sparse. Our new records along with reconstructions of deep water provenance from Nd isotopes constrain the timing and magnitude of past changes in AMOC from an additional northwestern Atlantic region forming a depth transect between 3000 and 4760 m water depth. Our extended and improved dataset confirms the weakening of the AMOC during deglacial cold spells such as Heinrich Event 1 and the Younger Dryas interrupted by a reinvigoration during the Bølling‐Allerød interstadial as seen in the prominent 231Pa/230Th records from the Bermuda Rise. However, in contrast to the Bermuda Rise records we find a clearly reduced circulation strength during the Last Glacial Maximum in the deep Atlantic

²³¹Pa/²³⁰Th records from the northwestern Atlantic covering the period since Heinrich Stadial 2 until today

In this dataset we present four new high resolution ²³¹Pa/²³⁰Th records from the Blake-Bahama Outer-Ridge (ODP sites 1059-1062) covering the last 30 ka. The so formed depth-transect (3000 – 4700 m water depth) is extended by the remeasured core KNR140 12JPC (4250 m water depth). The investigated ODP sites at the BBOR have also been analyzed for major element composition of the sediment by XRF scanning of discrete bulk sediment samples and concentration measurements of biogenic opal. We also present one new ¹⁴C age for GeoB 1515-1 and recalibrated ages for this core and GeoB 1523-1 in the equatorial western Atlantic. With these new age models for GeoB 1515-1 and GeoB 1523-1, ²³¹Pa/²³⁰Th records of these cores have slightly changed in timing and amplitude