The Evolution Of The Paleo-ocean Environment Revealed By Planktonic Foraminifera Since 70 Ka In The Mid-latitude North Atlantic

The mid-latitude North Atlantic(40-50°N),located between the subpolar gyre(SPG)and subtropical gyre(STG),is more sensitive to global climate change.It mainly hosts two different water masses namely cold and fresh water from polar-subpolar regions and warm and salty North Atlantic Current(NAC)due to its unique location.The NAC strengthens to the north in summer,so the upper water column is covered by warm and oligotrophic subtropical water masses,resulting in stratification and mixed-layer thinning.When the polar water inflow,the upper water column is covered by cold and high productive water mass,resulting in forming a thick and homogeneous mixed layer where has low temperature and salinity gradients.However,it is believed that the polar water periodically expanded from its modern location to the mid-latitudes during the glacial.The SPG may have strengthened when the North Atlantic current system moved to the southern and eastern.This phenomenon is attributed to the repeated collapse of the ice sheet around the North Atlantic,which produces large amount of meltwater,ultimately weakening the North Atlantic deep-water formation.Therefore,the paleoceanographic research in this area gradually came into intense focus.Samples were taken from the gravity core Hu71-377 in the ice rafted debris(IRD)belt,which was collected by the Dalhousie University cruise Hu71022.The age model(0?30 ka)of core Hu71-377 was established by measuring ¹⁴C age on Accelerator Mass Spectrometer(AMS¹⁴C)in this study.The rest of the age model(30?70 ka)is constructed by obtaining key tie points through tuning the N.pachyderma?¹⁸O curve to the?¹⁸O curve of the Greenland ice core and isotope stratigraphic records of N.pachyderma in subpolar core PS2644.This study analyzed multi-proxies,such as IRD,planktonic foraminiferal assemblages,?¹⁸O and?¹³C of Neogloboquadrina pachyderma and Neogloboquadrina incompta.This study also reconstructed the sea surface temperature(SST)and sea surface salinity(SSS)by making paired Mg/Ca ratios and oxygen isotopic measurements for the same species since the last 70 ka.The summary of this research is as follows:(1)Six Heinrich events were identified in core Hu71-377,that occurred at:?15.6ka,?22 ka,?29 ka,?38 ka,?46.5 ka and?62 ka,suggesting that the iceberg rafting and rapid climate changes significantly altered the sea-surface of the North Atlantic.H2 and H4 are characterized by the typical features of the H-events while a narrow H3 and H6IRD peak is contrary to the view of a lack of IRD peak in the eastern North Atlantic and southern margin of the IRD records.Further,two IRD peaks during H1indicate that there were at least two phases of ice sheet discharge.Therefore,the IRD discontinuity(?15.1 ka)between H1.1 and H1.2 may be caused by the meltwater and different ice streams during the deglacial period.(2)Different hydrological changes in the upper water column were recorded during H-events events.The?¹⁸O_N.pachyderma in H3 and H5 didn't show the same significant lighter?¹⁸O_N.pachyderma as it shown in other H-events,which may be due to the stronger impact of meltwater during H1,H2,H4 and H6.It could also have been impacted by more warm water inflow during the H3 and H5,thus weakening the meltwater effect.Mg/Ca ratios in Neogloboquadrina pachyderma and Neogloboquadrina incompta were converted to the sea-surface temperature during the last 70 ka.The results show that sea surface became cooling during H2.However,SST_N.pachyderma and SST_N.incompta increased significantly by 6?8?in H1 and H6.SST_N.pachyderma increased by 3.5°C in H4.It is worth noting that SST_N.incompta increased by 3°C in H3 and H5,which once again indicates the seasonal North Atlantic warm water input in H3 and H5.(3)N.pachyderma and N.incompta dominated the planktonic foraminiferal assemblages in core Hu71-377.The ratio of the relative abundance of N.incompta to N.pachyderma was calculated to infer the SST related to water masses in which<50%during MIS2 and MIS4,reflecting the colder environment by the polar water input.The ratio>50%during the warm-MIS3 and Holocene,which may reflect the warm and salty NAC inflow.However,due to the seasonal differences of planktonic foraminifera and changes in depth habitat may lead to changes in their relative abundance,therefore,the abundance ratio of N.incompta to N.pachyderma may overestimate the SST.Taking into account the limitation of the abundance ratios,the Mg/Ca proxy was used to reconstruct the SST.(4)Seawater oxygen isotopes(?¹⁸O_w)and sea-surface salinity(SSS)were reconstructed using the SST and oxygen isotopes in N.pachyderma and N.incompta.Due to the meltwater and warm NAC water interaction,a strong thermocline and halocline in the upper water column might have existed during the H3 as apparent from the SSS_N.pachyderma and SSS_N.incompta.At this time,the North Atlantic meridional overturning current(AMOC)might have remained strong.However,a significant reduction in the SSS_N.pachyderma?SSS_N.incompta was reconstructed during the H1,H2,H4and H6,indicating that the strong meltwater effect may have weakened or significantly weakened the AMOC.(5)The difference between SSS_N.pachydermaand SSS_N.incompta decreased,indicating that strong convective mixing while the difference between SST_N.pachyderma and SST_N.incompta increased in MIS4,reflecting surface cooling and mixing led to N.pachyderma inhabit deeper during the MIS4 in the upper water column.The thermocline might have disappeared in the warm-MIS3,but the homogeneous and thick mixed-layer gradually formed from the onset of cold-MIS3 to MIS2.The SST and SSS are negatively correlated during MIS1,global sea level rise may explain SSS decrease.(6)The?¹³C shows that the?¹³C in MIS3 and MIS1 are heavier,indicating that the air-sea exchange effect was better,while the air-sea exchange in MIS4 and MIS2were weakened.The difference between?¹³C_N.pachyderma and?¹³C_N.incomptamay mainly reflect the paleo-environment during H-events.The difference between?¹³C_N.pachydermaand?¹³C_N.incompta decreased in H1,H2 and H6,which may have been affected by the deepening of the thermocline or the vertical mixing of seawater driven by strong winds.However,their?¹³C difference increased in H4 and H5,indicating that the seasonal thermocline became shallow related to the NAC enhancement and water stratification during the summer.In addition,?¹³C_N.pachyderma has no significant correlation with shell's weight;on the contrary,the weight of N.incompta and?¹³C_N.incompta are positively correlated during the cold LGM and negatively correlated during the warm Holocene,indicating that shell's weight may cause the?¹³C offset of the shell?¹³C and seawater?CO₂.Therefore,it is not ruled out that shell's weight increase during H-events may lead to?¹³C_N.incompta become heavier,resulting in an increase in the difference between?¹³CN.pachyderma and?¹³CN.incompta.