Hydrographic Characteristics and Oceanic Heat Flux in the Upper Arctic Ocean over the Alpha Ridge Observed by the DTOP Platform in 2018 and 2021

In 2018 and 2021,the Drift-Towing Ocean Profilers(DTOP)provided extensive temperature and salinity data on the upper 120m ocean through their drifts over the Alpha Ridge north of the Canada Basin.The thickness and temperature maximum of Alaska Coastal Water(ACW)ranged from 20m to 40m and-1.5℃to-0.8℃,respectively,and the salinity generally maintained from 30.2 to 32.5.Comparison with World Ocean Atlas 2018’s climatology manifested a 40m-thick and warm ACW roughly ex-ceeding the temperature maximum by 0.4–0.5℃in June–August 2021.This anomalously warm ACW was highly related to the ex-pansion of the Beaufort Gyre in the negative Arctic Oscillation phase.During summer,the under-ice oceanic heat flux F_(w)^(OHF)was elevated,with a maximum value of above 25Wm^(-2).F_(w)^(OHF)was typically low in the freezing season,with an average value of 1.2Wm^(-2).The estimates of upward heat flux contributed by ACW to the sea ice bottom F_(w)^(OHF)were in the range of 3–4Wm^(-2)in June–August 2021,when ACW contained a heat content of more than 80MJm^(-2).The heat loss over this period was driven by a weak stratification upon the ACW layer associated with a surface mixed layer(SML)approaching the ACW core.After autumn,F_(w)^(OHF)was reduced(<2 Wm^(-2))except during rare events when it elevated F_(w)^(OHF)slightly.In addition,the intensive and widespread Ekman suction,which created a violent upwelling north of the Canada Basin,was largely responsible for the substantial cooling and thinning of the ACW layer in the summer of 2021.

Late Quaternary terrigenous sedimentation in the Western Arctic Ocean as exemplified by a sedimentary record from the Alpha Ridge

Terrigenous components in sediment core B84A from the Alpha Ridge, Western Arctic Ocean, have been investigated to reconstruct Mid to Late Quaternary variations in sedimentation, provenance, and related climate changes. The core stratigraphy, evaluated by a combination of variations in Mn content, color cycles, foraminiferal abundance, and lithological correlation, extends back to estimated Marine Isotope Stage 12. Twelve Ice Rafted Detritus （IRD, 〉250 ttm） events were identified and interpreted to mostly occur during deglaciation. The Canadian Arctic, which was covered by ice sheets during glacial periods, is suggested to be the major source region. The IRD events likely indicate the collapses of ice sheets, possibly in response to abrupt climate changes. Grain size analysis of B84A indicates sedimentologically sensitive components in core B84A in the 4 9 #m and 19 53/~m silt subfractions, which are inferred to be mainly transported by currents and sea ice, respectively. Down core variability of these two fractions may indicate changes in ice drift and current strength. In accordance with previous studies in the central Arctic Ocean, the average sedimentation rate in core B84A is about 0.4 cm.ka-1. Compared with the relatively high sedimentation rates on the margins, sedimentation in the central Arctic Ocean is limited by sea ice cover and the correspondingly low bioproductivity, as well as the long distance from source regions of terrigenous sediment.