Late Pleistocene and Holocene benthic and planktonic foraminifera from the central Lomonosov Ridge

Presently available bathymetric charts show that the area between about 88°15’–89°N and 140°–180°E of the Lomonosov Ridge is characterized by a >1000 m deep depression in the ridge morphology forming a local "intra basin". During the 2005 Beringia/Healy-Oden Trans-Arctic EXpedition (HOTRAX), a detailed investigation of this part of the Lomonosov Ridge was conducted including multibeam bathymetric mapping, subbottom profiling, hydrographic measurements and sediment coring (Darby et al., 2005). The subbottom profiles reveal an expanded sediment stratigraphy in the intra basin suggesting that the area have acted as a local sediment trap with higher sediment accumulation rates compared to the 1000 m shallower ridge crest (Bjork et al., in press). Core HLY0503-18JPC, investigated in this study, was retrieved at 2598 m water depth from the floor of the intra basin. The top 68 cm of this 12 m long sediment core contain abundant calcareous nanno- and microfossils. For this study, the content of benthic and planktonic foraminifera was quantified. The number of specimens per gram sediment showed several major peaks and valleys. Peak abundances of benthic and planktonic foraminifera coincide well with each other. The largest peak occurs near the base of this 68 cm long calcareous-rich sequence. All calcareous microfossils are well preserved, showing no signs of dissolution in the larger grain size fraction (>150 m). Assemblage analyses of four samples, of the size fraction >150 m, show a dominant abundance of Neogloboquadrina pachyderma (sinistral), accounting for up to 99% of the assemblage. There is no complete age model yet established for core HLY0503-18JPC, although 14C dating made on tube worms and bryozoan mats show ages of 35.200 ± 1500 (radiocarbon yrs) at 28-29 cm core depth and 32.800 ± 1500 (radiocarbon yrs) at 39 cm indicating sedimentation rates of about 1 cm/kyr. These 14C ages are in stark contrast to the nannofossil stratigraphy where the most nannofossil rich samples found between 42 and 56 cm suggest a Holocene age (Fornaciari et al., 2006) implying several times higher sedimentation rates. Additional 14C dating will be carried out on planktonic and benthic foraminifera as a next step in this study.