Enhanced storm activities triggered the North Pacific deep convection during the Younger Dryas event

<p>The occurrence of deep convection could redistribute ocean heat and materials, and induce robust climate and biogeochemical changes. The convection in the North Pacific is quite shallow now (typically 300-m), but paleo records and model simulations suggest that it might reach 2000-3000 m during stadials in the last deglaciation, such as the Heinrich event 1 (H1: ~17.5-15 ka) and Younger Dryas event (YD: ~12.8-11.5 ka). The deep convection during H1 has been explained by increased North Pacific surface salinity due to evaporation and precipitation changes, but this explanation conflicts with many paleo records for YD. Here we collected published paleo records in the northwest Pacific and carried out simulations for the YD period. We show that due to the weakened Atlantic Meridional Overturning Circulation (MOC) during YD, the oceanic Meridional Heat Transport (MHT) weakened. According to the Bjerknes compensation, the atmospheric MHT strengthened. Because atmospheric MHT mainly occurs through baroclinic eddies in extratropics, storm activities strengthened. The strengthened wintertime storm activities induced more oceanic turbulent heat loss and triggered deep convections in the North Pacific, and further contributed to a seesaw pattern of MOC strengths between the North Pacific and North Atlantic. Our result not only provides a new explanation for the North Pacific deep convection during YD but also suggest that synoptic-scale atmospheric variations are capable of influencing low-frequency paleoclimate changes.</p>

MlCROPALEODOLOGICAL STUDY OF CORE Z1 FROM IONIAN SEA PALEOCLIMATOLOGICAL- PALEOOICOLOGICAL CONCLUSIONS

The preliminary results based on the abundances of planktonic foraminifera and oxygen isotopes at the sediments of the core Z1, selected from the Ionian Sea (Otranto Basin) revealed three main climatic periods: Last Glacial, Late Glacial and Holocene. The warmest conditions of the studied interval prevailed during the formation of sapropel S1. Four stadials occurred during Late Glacial and Holocene. The older corresponds to Younger Dryas event, while the other three occurred during Holocene. The establishment of warm and low salinity surficial waters seems that caused the reduction of water mass circulation and thus the formation of the sapropel S1. Although the presence of a stadial around the middle of the S1, no distinct interruption of the sapropelic sedimentation have been occurred. The younger stadial coincides with the end of the sapropel S1. The evolution of the planktonic assemblages seems to be similar with that of the Central Mediterranean for the studied interval. Therefore, almost all the biozones and ecozones of the Central Mediterranean have been recognized at the sediments of Z1. The application of the factor analysis revealed four factors. The first two correspond to the variation of SST, while the other two are related to the development of high fertility waters during cold periods and during the formation of S1.

Wood14C Dating with AixMICADAS: Methods and Application to Tree-Ring Sequences from the Younger Dryas Event in the Southern French Alps

The AixMICADAS facility is in part dedicated to research on radiocarbon (14C) calibration by means of various archives. For this purpose, we are improving upon the capacity to accurately date subfossil wood. In the current study, nine chemical pretreatment protocols are tested on six wood samples of known ages. The optimization based on14C ages,13C/12C ratios, carbon % and overall mass yield % leads us to favor the acid-base-acid-bleaching pretreatment (ABA-B). This efficient method is shown to provide a residue of holocellulose with optimal blanks equivalent to an age of 51,30014C BP with a standard deviation of 1500 yr based on 25 analyses. The seven wood samples from the Sixth International Radiocarbon Intercomparison (SIRI) are then analyzed as a further verification of the accuracy of our method. As a first scientific contribution, we studied two tree-ring sequences from subfossil pines (Barb12 and Barb17) collected in the southern French Alps. New14C analyses were performed at high resolution (every third year) and are shown to agree well with results obtained previously by high precision β-counting on CO2from large samples at lower resolution for Barb17 and accelerator mass spectrometry (AMS) data for Barb12. The new14C series are then matched to the Kauri and YDB chronologies: the new sequence of Barb12-17 tentatively corresponds to the interval between 12,836 and 12,594 cal BP within the Younger Dryas cold period. The14C comparison between the Barb12-17 sequence from France and the Kauri sequence from New Zealand allows calculating the14C Inter-Hemispheric Gradient (IHG), with an average value of ca. 57 yr. The IHG stayed relatively high throughout the studied period. Interestingly, the IHG exhibits a transient maximum value (ca. 100 yr) during the period of rapid Δ14C rise (12,750–12,720 cal BP), a behavior that could be due to a delayed response of the Southern Hemisphere.

Deglacial sea level history of the East Siberian Sea and Chukchi Sea margins

Deglacial (12.8–10.7 ka) sea level history on the East Siberian continental shelf and upper continental slope was reconstructed using new geophysical records and sediment cores taken during Leg 2 of the 2014 SWERUS-C3 expedition. The focus of this study is two cores from Herald Canyon, piston core SWERUS-L2-4-PC1 (4-PC1) and multicore SWERUS-L2-4-MC1 (4-MC1), and a gravity core from an East Siberian Sea transect, SWERUS-L2-20-GC1 (20-GC1). Cores 4-PC1 and 20-GC were taken at 120 and 115 m of modern water depth, respectively, only a few meters above the global last glacial maximum (LGM; ∼ 24 kiloannum or ka) minimum sea level of ∼ 125–130 meters below sea level (m b.s.l.). Using calibrated radiocarbon ages mainly on molluscs for chronology and the ecology of benthic foraminifera and ostracode species to estimate paleodepths, the data reveal a dominance of river-proximal species during the early part of the Younger Dryas event (YD, Greenland Stadial GS-1) followed by a rise in river-intermediate species in the late Younger Dryas or the early Holocene (Preboreal) period. A rapid relative sea level rise beginning at roughly 11.4 to 10.8 ka ( ∼ 400 cm of core depth) is indicated by a sharp faunal change and unconformity or condensed zone of sedimentation. Regional sea level at this time was about 108 m b.s.l. at the 4-PC1 site and 102 m b.s.l. at 20-GC1. Regional sea level near the end of the YD was up to 42–47 m lower than predicted by geophysical models corrected for glacio-isostatic adjustment. This discrepancy could be explained by delayed isostatic adjustment caused by a greater volume and/or geographical extent of glacial-age land ice and/or ice shelves in the western Arctic Ocean and adjacent Siberian land areas.

Deglacial sea-level history of the East Siberian Sea Margin

Abstract. Deglacial (12.8–10.7 ka) sea-level history on the East Siberian continental shelf/upper continental slope was reconstructed using new geophysical records and sediment cores taken during Leg 2 of the 2014 SWERUS-C3 expedition. The focus of this study is two cores from Herald Canyon, piston core SWERUS-L2-4-PC1 (4-PC) and multicore SWERUS-L2-4-MC1 (4-MC1) and a gravity core from an East Siberian Sea Transect, SWERUS-L2-20-GC1 (20-GC). Cores 4-PC1 and 20-GC were taken at 120 m and 115 m modern water depth, respectively, only a few meters above the global last glacial maximum (LGM, ~ 24 kiloannum (ka)) minimum sea level of ~ 125–130 meters below sea level (mbsl). Using calibrated radiocarbon ages mainly on molluscs for chronology and the ecology of benthic foraminifera and ostracode species to estimate paleo-depths, the data reveal dominance of river-proximal species during the early part of the Younger Dryas event (YD, Greenland Stadial GS-1) followed by a rise in river-intermediate species in the late Younger Dryas or the early Holocene (Preboreal) period. A rapid relative sea-level rise beginning roughly 11.4 to 10.8 ka (~ 400 cm core depth) during is indicated by a sharp faunal change and unconformity or condensed zone of sedimentation. Regional sea level at this time was about 108 mbsl at the 4-PC1 site and 102 mbsl at 20-GC. Regional sea-level during the YD was about 40 to 50 meters lower than those predicted by geophysical models corrected for glacio-isostatic adjustment. This discrepancy could be explained by delayed isostatic adjustment caused by a greater volume and/or geographical extent of glacial-age land ice and/or ice shelves in the western Arctic Ocean and adjacent Siberian land areas.