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FP5

PACLIVA Report Summary

Project ID: EVK2-CT-2002-00143
Funded under: FP5-EESD
Country: United Kingdom

Deep water palaeoreconstructions from N. Atlantic

Deep water hydrological characteristics have been determined for three cores in the North Atlantic, HM03-133-25 from the Faroe-Shetland Channel, MD95 2024 from Orphan Knoll in the southern Labrador Sea, and ENAM9606 from the Feni Drift in the Rockall Trough.

Gravity core HM03-133-25 was collected from the southern end of Faroe-Shetland Channel (60° 07, 06° 04'W, 1156 m depth). The site is swept by relatively strong currents, yielding a high percentage of sand (20-60%), with a high of over 70% during the Bølling-Allerød (B-A). This 4.7 m long core has been dated by AMS C-14 and by correlation to core MD99-2284 from the northern entrance to Faroe Shetland Channel. The core spans the last 14-20 ka (there is uncertainty in the age of the base) with sedimentation rates relatively low in the late glacial (10 cm/ka) and high in the Holocene (early Holocene 65 cm/ka decreasing to 12cm/ka in the late Holocene). Flow speed has been monitored by current meters and ADCP for several years near the core site. Speed, temperature and salinity data reveal fast flows in both warm saline Atlantic inflow and cold less saline outflow.

The magsus signature in cores HM03 and MD99 cores correlate during the B-A, Younger Dryas (YD) and 8.2 ka event. The magnetic mineralogy is presently being investigated. The 8.2 ka event does not appear in the isotopic records.

The cores were scanned for magnetic susceptibility and for spectral colour reflectance. The fine fraction <63 µm was analysed for the mean grain size of the non-biogenic sortable silt fraction (abbreviated as SS) (mean of 10-63 µm range), with carbonate and biogenic opal removed. The grain size distribution was measured with a Coulter Counter.

The sand percentage is regarded here as an indicator of flow speed as the faster flows through the channel are capable of transporting sand and preventing deposition of mud. The drop in sand to 14% around the 8.2 event time indicates relatively sluggish flow, while the peak over 80% in the B-A records fast flows under warm conditions. Holocene speeds increase after the 8.2 event up to the present. The YD does not show slow flow. A peak in flow speed is recorded between 7 and 6 ka and a plateau at 8 to 7 ka rising from the 8.2ka low. The period 0-2000 years BP is marked by high flow speeds. The sortable silt data are very noisy, and difficult to interpret as the silt is trapped as an interstitial component in sand. SS is lower over the last 2000 years than at other times.

NEAP 15K/16B located at 2850 m on Gardar Drift is influenced by Iceland-Scotland Overflow Water (ISOW); while MD95-2024 at 3539 m from the Labrador Sea (Orphan Knoll), is affected by Denmark Strait Overflow Water (DSOW). Spectral analyses reveal similar pacings in the millennial band of ~1.5 ka, but deep water flow trends at centennial to millennial time-scales are opposing. This may be explained by variations in either the flux or/and the density structure of the lower water column. The latter is unlikely, since bottom water temperatures remained fairly uniform during the Holocene, suggesting a persistent dominance of DSOW at Orphan Knoll.

Rockall Trough is a distal site along the flow path of the North Atlantic thermohaline circulation (THC), bottom waters are dominantly composed of recirculated North Atlantic Deep Water entering from the southeast. Sustained bottom current activity resulted in the formation of Feni Drift, one of the major drift deposits in the North Atlantic. This area will be capable of recording even relatively minor fluctuations of bottom current activity precisely because it constitutes a distal setting.

The mean size of the sortable silt fraction is a proxy for relative bottom current speed. We apply this proxy on Feni Drift core ENAM9606 (55º39 N 14ºW, 2543m depth). We investigated the Holocene, at bicentennial or higher resolution. The record can be divided into four main intervals. The early Holocene displays a continuous decrease in SS mean size and inferred bottom current speed, culminating in a sustained minimum between 9.5 and 8ka BP. The mid-Holocene interval (7.7-5.6ka) is characterized by strong and stable bottom currents, punctuated by a single minimum around 7ka. This is followed by slightly weaker and more variable flow speeds until 3.4ka. The late Holocene (<3.4ka) shows drastically enhanced variability including several rapid drops in flow speed. The interval from 1.5- 0.9ka shows particularly marked fluctuations; here abrupt SS mean size minima alternate with maximum values comparable to the rather stable early mid-Holocene. Finally, over the following two centuries a continuous decrease in SS mean size finishes with moderately low values in the remainder of the record.

Reported by

Department of Earth Sciences, University of Cambridge
Downing Street
CB2 3EQ Cambridge
United Kingdom
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