Diatom abundance in middle-to-late Pleistocene sediments of IODP Site 303-U1304

We examined diatom assemblages in a series of remarkable laminated diatomaceous ooze (LDO) horizons in the marine sediments from Integrated Ocean Drilling Program (IODP) Site U1304 to reconstruct the middle-to-late Pleistocene paleoceanographic evolution of the northern North Atlantic Ocean. Four confirmed diatom biohorizons combined with calcareous nannofossil and paleomagnetic stratigraphies established the chronological framework for the material. The planktonic, araphid, needle-like species Thalassiothrix longissima was the greatest contributor to the LDO facies. From the results of a principal component analysis using the percent abundances of 65 significant (p = 5%) diatom taxa, except for Tx. longissima, which was extremely dominant in almost all horizons observed, we identified two principal component (PC) axes. Taxa probably associated with the stratigraphic distribution of the major zonal marker Neodenticula seminae (ranging from 1.26 to 0.84 Ma in this ocean) loaded on PC1 with a high value. PC2 was related to the ocean surface temperature. The stratigraphic variability of the PC2 score indicated that switching between warm- and cold-water assemblages occurred concurrently with LDO deposition (or extreme Tx. longissima dominance) episodes in several horizons (particularly after 0.84 Ma), suggesting that the Subarctic Convergence (SAC) oceanic front passed over Site U1304 during Pleistocene glacial/interglacial cycles. Our floral evidence supports the model of nearly monospecific LDO formation caused by the enhanced physical accumulation of particular diatoms such as Tx. longissima. On the other hand, Nd. seminae, which probably contributes to spring phytoplankton blooms in the modern ocean, was present only between 1.26 and 0.84 Ma in this area. Thus, we infer that the main contributor of export flux in the regional annual primary production cycle would have shifted drastically from one of a spring phytoplankton bloom leader (Nd. seminae) to minor but mass dump assemblages (Tx. longissima etc.) in the mid-Pleistocene.

North Atlantic IODP Site 303-U1308 Middle Pleistocene Heinrich Events record

Heinrich events are well documented for the last glaciation, but little is known about their occurrence in older glacial periods of the Pleistocene. Here we report scanning XRF and bulk carbonate d18O results from Integrated Ocean Drilling Program Site U1308 (reoccupation of Deep Sea Drilling Project Site 609) that are used to develop proxy records of ice-rafted detritus (IRD) for the last ~1.4 Ma. Ca/Sr is used as an indicator of IRD layers that are rich in detrital carbonate (i.e., Heinrich layers), whereas Si/Sr reflects layers that are poor in biogenic carbonate and relatively rich in detrital silicate minerals. A pronounced change occurred in the composition and frequency of IRD at ~640 ka during marine isotope stage (MIS) 16, coinciding with the end of the middle Pleistocene transition. At this time, "Hudson Strait" Heinrich layers suddenly appeared in the sedimentary record of Site U1308, and the dominant period of the Si/Sr proxy shifted from 41 ka prior to 640 ka to 100 ka afterward. The onset of Heinrich layers during MIS 16 represents either the initiation of surging of the Laurentide Ice Sheet (LIS) off Hudson Strait or the first time icebergs produced by this process survived the transport to Site U1308. We speculate that ice volume (i.e., thickness) and duration surpassed a critical threshold during MIS 16 and activated the dynamical processes responsible for LIS instability in the region of Hudson Strait. We also observe a strong coupling between IRD proxies and benthic d13C variation at Site U1308 throughout the Pleistocene, supporting a link between iceberg discharge and weakening of thermohaline circulation in the North Atlantic.