231Pa/230Th in the northwestern Atlantic: circulation versus particles?

2020 ◽  
Author(s):  
Finn Süfke ◽  
Frerk Pöppelmeier ◽  
Patrick Blaser ◽  
Jörg Lippold
Keyword(s):  
North Atlantic ◽  
Deep Sea ◽  
Potential Effect ◽  
The Past ◽  
The North ◽  
Circulation Change ◽  
Sedimentary Core ◽  
Bermuda Rise ◽  
The Last Glacial Maximum ◽  
The Last Glacial

<p>In 2004 McManus et al. published their famous <sup>231</sup>Pa/<sup>230</sup>Th record from the Bermuda Rise revealing millennial-scale changes in circulation strength back to the Last Glacial Maximum. This record marks the boost of this proxy as a kinematic circulation change proxy for the Atlantic Ocean and the initial rising slope on the ‘Elderfield-Curve’. However, the up-to-date data base of Atlantic sedimentary <sup>231</sup>Pa/<sup>230</sup>Th records gives a rather inconsistent picture of changes in the circulation strength in the Atlantic throughout the past 25 ka (Ng et al., 2018). Since both radioisotopes are strongly particle reactive it is obvious that scavenging processes may play a major role in their cycling as well. At ocean margins such processes do have a major impact on <sup>231</sup>Pa/<sup>230</sup>Th, leading to increased values and thus potentially overprinting the circulation signal. In contrast, records from open ocean sites are assumed to show a less biased circulation signal. In addition, the GEOTRACES program (Schlitzer et al., 2018) provided valuable seawater data allowing for examining the cycling of both radioisotopes under today’s circulation regime in more detail. A transect across the North Atlantic by Hayes et al. (2015) revealed that nepheloid layers contribute to strong bottom scavenging of <sup>231</sup>Pa and <sup>230</sup>Th in the northwestern Atlantic basin. Surprisingly, sedimentary core-top values do not mirror predominant scavenging effects but rather indicate a strong export of <sup>231</sup>Pa and therefore a circulation signal. With our modern proxy toolbox, it is impossible to reconstruct the occurrence and intensity of past nepheloid layers and hence their potential effect on recorded <sup>231</sup>Pa/<sup>230</sup>Th variations. Therefore, isotope-enabled models may help to better decipher the interwoven processes controlling <sup>231</sup>Pa/<sup>230</sup>Th (Rempfer et al., 2017; Lerner et al., 2019). Here an up-to-date compilation of northwestern Atlantic <sup>231</sup>Pa/<sup>230</sup>Th data will be presented. Our findings base on records covering the last 25 ka and will be interpreted in the context of recent model simulations as well as compared to seawater data. Thus, we aim for a deeper understanding of <sup>231</sup>Pa and <sup>230</sup>Th cycling in the northwestern Atlantic.</p><p>References:</p><p>Hayes, C., et al. (2015), Deep-Sea Res. Pt. II, 116, 29-41.<br>Lerner et al. (2020), Deep Sea. Res. Pt. I, 155, 1-41.<br>McManus, J. F., et al. (2004), Nature, 428, 834-837.<br>Ng, H., et al. (2018), Nat. Comm., 9, 1-10.<br>Rempfer et al. (2017), EPSL, 468, 27-37.<br>Schlitzer, R., et al. (2018), Chem. Geol., 493, 210-223.</p><p> </p>

Climatic Record of the Past 130,000 Years in North Atlantic Deep-Sea Core V23-82: Correlation with the Terrestrial Record

1973 ◽  
Vol 3 (1) ◽  
pp. 110-116 ◽  
Author(s):  
Constance Sancetta ◽  
John Imbrie ◽  
N.G. Kipp
Keyword(s):  
North Atlantic ◽  
Deep Sea ◽  
Planktonic Foraminifera ◽  
Linear Interpolation ◽  
Stratigraphic Correlation ◽  
Last Glacial ◽  
The Past ◽  
The North ◽  
The North Atlantic ◽  
The Last Glacial

AbstractQuantitative paleo-environmental analyses of planktonic foraminifera in 182 samples covering the past 130,000 years in North Atlantic deep-sea core V23-82 yield time series interpreted in terms of changing surface-water conditions. An absolute chronology is estimated by linear interpolation between levels dated by 14C or by stratigraphic correlation with other radiometrically dated climatic records. Significant events include: (1) rapid warming at 127,000 YBP, marking the start of the penultimate North Atlantic and European interglacial; (2) 124,000 YBP temperature maximum (Eemian); (3) 109,000 YBP cooling, correlated with the beginning of the last European glaciation (Würm), and representing a temporary cooling of the North Atlantic; (4) severe cooling 73,000 YBP, marking the start of the last full glacial regime in the North Atlantic; (5) short warm intervals within the last glacial regime dated at 59,000 YBP, 48,000 YBP, and 31,000 YBP; (6) rapid termination of the last glacial interval at 11,000 YBP; (7) a 6000 YBP hypsi-saline, followed by lowering salinity values presumably associated with decreasing flux of Gulf Stream waters over the core site.

scholarly journals Stationary Wave Reflection as a Mechanism for Zonalizing the Atlantic Winter Jet at the LGM

2016 ◽  
Vol 73 (8) ◽  
pp. 3329-3342 ◽  
Author(s):  
Marcus Löfverström ◽  
Rodrigo Caballero ◽  
Johan Nilsson ◽  
Gabriele Messori
Keyword(s):  
North Atlantic ◽  
Wave Breaking ◽  
Planetary Wave ◽  
Wave Reflection ◽  
Stationary Wave ◽  
Laurentide Ice Sheet ◽  
The North ◽  
Nonlinear Reflection ◽  
The Last Glacial Maximum ◽  
The Last Glacial

Abstract Current estimates of the height of the Laurentide Ice Sheet (LIS) at the Last Glacial Maximum (LGM) range from around 3000 to 4500 m. Modeling studies of the LGM, using low-end estimates of the LIS height, show a relatively weak and northeastward-tilted winter jet in the North Atlantic, similar to the modern jet, while simulations with high-end LIS elevations show a much more intense and zonally oriented jet. Here, an explanation for this response of the Atlantic circulation is sought using a sequence of LGM simulations spanning a broad range of LIS elevations. It is found that increasing LIS height favors planetary wave breaking and nonlinear reflection in the subtropical North Atlantic. For high LIS elevations, planetary wave reflection becomes sufficiently prevalent that a poleward-directed flux of wave activity appears in the climatology over the midlatitude North Atlantic. This entails a zonalization of the stationary wave phase lines and thus of the midlatitude jet.

The Magnitude and Proximate Cause of Ice-Sheet Growth Since 35,000 yr B.P.

2001 ◽  
Vol 56 (3) ◽  
pp. 299-307 ◽  
Author(s):  
Isaac J. Winograd
Keyword(s):  
North Atlantic ◽  
Ice Sheet ◽  
Proximate Cause ◽  
Ice Volume ◽  
The North ◽  
Data Compilation ◽  
Sst Warming ◽  
Late Wisconsinan ◽  
The Last Glacial Maximum ◽  
The Last Glacial

AbstractThe magnitude of late Wisconsinan (post-35,000 yr B.P.) ice-sheet growth in the Northern Hemisphere is not well known. Ice volume at ∼35,000 yr B.P. may have been as little as 20% or as much as 70% of the volume present at the last glacial maximum (LGM). A conservative evaluation of glacial–geologic, sea level, and benthic δ18O data indicates that ice volume at ∼35,000 yr B.P. was approximately 50% of that extant at the LGM (∼20,000 yr B.P.); that is, it doubled in about 15,000 yr. On the basis of literature for the North Atlantic and a sea-surface temperature (SST) data compilation, it appears that this rapid growth may have been forced by low-to-mid-latitude SST warming in both the Atlantic and Pacific Oceans, with attendant increased moisture transport to high latitudes. The SST ice-sheet growth notion also explains the apparent synchroneity of late Wisconsinan mountain glaciation in both hemispheres.

scholarly journals Simulated stability of the Atlantic Meridional Overturning Circulation during the Last Glacial Maximum

2021 ◽  
Vol 17 (2) ◽  
pp. 615-632
Author(s):  
Frerk Pöppelmeier ◽  
Jeemijn Scheen ◽  
Aurich Jeltsch-Thömmes ◽  
Thomas F. Stocker
Keyword(s):  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Bering Strait ◽  
Deep Water Formation ◽  
Overturning Circulation ◽  
The North ◽  
Water Formation ◽  
Meridional Overturning ◽  
The Last Glacial Maximum ◽  
The Last Glacial

Abstract. The response of the Atlantic Meridional Overturning Circulation (AMOC) to freshwater perturbations critically depends on its mean state. Large swaths of icebergs melting in the North Atlantic during the last deglaciation constituted such perturbations and can, thus, provide important constraints on the stability of the AMOC. However, the mean AMOC state during the Last Glacial Maximum (LGM), preceding the rapid disintegration of the ice sheets during the deglaciation, as well as its response to these perturbations remain debated. Here, we investigate the evolution of the AMOC as it responds to freshwater perturbations under improved LGM boundary conditions in the Bern3D intermediate complexity model. Particularly, we consider the effect of an open versus a closed Bering Strait and the effect of increased tidal dissipation as a result of the altered bathymetry due to the lower glacial sea level stand. The vigorous and deep AMOC under these glacial boundary conditions, consistent with previous simulations with different models, reacts more strongly to North Atlantic freshwater forcings than under preindustrial conditions. This increased sensitivity is mostly related to the closed Bering Strait that cuts off the freshwater escape route through the Arctic into the Pacific, thereby facilitating faster accumulation of freshwater in the North Atlantic and halting deep-water formation. Proxy reconstructions of the LGM AMOC instead indicate a weaker and possibly shallower AMOC than today, which is in conflict with the particularly strong and deep circulation states coherently simulated with ocean circulation models for the LGM. Simulations with reduced North Atlantic deep-water formation, as a consequence of potentially increased continental runoff from ice sheet melt and imposed changes in the hydrological cycle, more closely resemble the overturning circulation inferred from proxies. These circulation states also show bistable behavior, where the AMOC does not recover after North Atlantic freshwater hosing. However, no AMOC states are found here that either comprise an extreme shoaling or vigorous and concurrent shallow overturning as previously proposed based on paleoceanographic data.

Luminescence age constraints on the Pleistocene-Holocene transition recorded in loess sequences

2020 ◽  
Author(s):  
Daniela Constantin ◽  
Stefana-Madalina Sacaciu ◽  
Viorica Tecsa ◽  
Anca Avram ◽  
Robert Begy ◽  
...  
Keyword(s):  
Magnetic Susceptibility ◽  
North Atlantic ◽  
Ice Core ◽  
Soil Formation ◽  
Luminescence Dating ◽  
The Holocene ◽  
The Past ◽  
The North ◽  
The North Atlantic ◽  
The Last Glacial

<p>Here we investigate the timing of the last glacial loess - Holocene soil transition recorded in loess-paleosol sequences across the Chinese Loess Plateau, the SE European loess belt and the Central Great Plains, Nebraska, USA by applying comparative luminescence dating techniques on quartz and feldspars. Equivalent dose measurements were carried out using the single-aliquot regenerative-dose (SAR) protocol on silt (4–11 μm) and sand-sized (63–90 μm and coarser fraction when available) quartz. Feldspar infrared stimulated luminescence (IRSL) emitted by 4–11 μm polymineral grains was measured using the post IR-IRSL<sub>290</sub> technique.</p><p>The paleoenvironmental transition from the last glacial loess to the current interglacial soil was characterized using magnetic susceptibility and its frequency dependence. Based on the OSL ages and the threshold of the magnetic signal enhancement the onset of soil formation started around Termination 1 (~17 ka in the North Atlantic) as observed in radiocarbon-dated regional benthic δ<sup>18</sup>O stacks (Stern and Lisiecki, 2014) but before the stratigraphic Pleistocene/Holocene transition dated at 11.7 ka in ice core records (Svensson et al., 2008).</p><p>No major hiatuses in ages are identified in the investigated sites. A change in the sedimentation rate is generally observed at the Pleistocene-Holocene transition and no significant sedimentation change during the Holocene. Sedimentation rates of around 6 cm/ka are determined for the Holocene soil in most of the sites investigated.</p><p>The magnetic susceptibility indicates a gradual increase in pedogenesis after Termination 1 (∼17 ka in the North Atlantic). Based on this, we infer that the upbuilding soil formation prevailed over topdown soil formation during the Pleistocene-Holocene transition in the investigated sites (Roberts, 2008).</p><p> </p><p>References</p><p>Roberts, H.M., 2008. The development and application of luminescence dating to loess deposits: a perspective on the past, present and future. Boreas 37, 483-507.</p><p>Svensson, A., Andersen, K.K., Bigler, M., Clausen, H.B., Dahl-Jensen, D., Davies, S.M., Johnsen, S.J., Muscheler, R., Parrenin, F., Rasmussen, S.O., Röthlisberger, R., Seierstad, I., Steffensen, J.P., Vinther, B.M., 2008.A 60 000 year Greenland stratigraphic ice core chronology. Climate of the Past 4, 47-57.</p><p>Stern, J.V., Lisiecki, L.E., 2014. Termination 1 timing in radiocarbon-dated regional benthic δ18O stacks. Paleoceanography 29, 1127-1142.</p><p> </p><p>This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme ERC-2015-STG (grant agreement No [678106]).</p>

On thin ice: problems with Stanford and Bradley's proposed Solutrean colonisation of North America

Antiquity ◽  
2014 ◽  
Vol 88 (340) ◽  
pp. 606-613 ◽  
Author(s):  
Michael J. O'Brien ◽  
Matthew T. Boulanger ◽  
Mark Collard ◽  
Briggs Buchanan ◽  
Lia Tarle ◽  
...  
Keyword(s):  
North America ◽  
Iberian Peninsula ◽  
North Atlantic ◽  
Food Supply ◽  
Ice Shelf ◽  
Southern France ◽  
The North ◽  
The North Atlantic ◽  
The Last Glacial Maximum ◽  
The Last Glacial

Across Atlantic ice: the origin of America's Clovis culture(Stanford & Bradley 2012) is the latest iteration of a controversial proposal that North America was first colonised by people from Europe rather than from East Asia, as most researchers accept. The authors, Dennis Stanford and Bruce Bradley, argue that Solutrean groups from southern France and the Iberian Peninsula used watercraft to make their way across the North Atlantic and into North America during the Last Glacial Maximum (LGM). According to Stanford and Bradley, this 6000km journey was facilitated by a continuous ice shelf that provided fresh water and a food supply.AcrossAtlantic ice has received a number of positive reviews. Shea (2012: 294), for example, suggests that it is “an excellent example of hypothesis-building in the best tradition of processual archaeology. It challenges American archaeology in a way that will require serious research by its opponents”. Runnels (2012) is equally enthusiastic.

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