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Surface water palaeoreconstructions from N. Atlantic

Sea surface 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, using Mg/Ca (SST) and oxygen isotope (18Oforam which includes SST and 18Osw components) measurements on the planktonic foraminiferal species Globigerinoides bulloides. This surface dwelling species was abundant at all sites during both periods of interest (8000-6000 and 2000-0 years BP).

Reconstructions salinity at Femi Drift are qualitatively consistent with multidecadal variability in the instrumental record, notably lower salinity in the 1920s and 1970s, but the amplitude of the variability is overestimated by a factor of 5. For the 20th century, both Mg/Ca- and 18Oc-derived calcification temperatures (the latter based on assumed 18Ow = 0.50 vs. V-SMOW) are consistent with late spring to early summer (May-June) instrumental temperatures. However, the two proxies yield divergent results, implying temporally variable18Ow, presumably due to changing surface salinities.

Multidecadal to century-scale variability is common throughout for T(Mg/Ca), ?18Oc and derived ?18Ow records at Femi Drift implying recurrent hydrographic shifts and no long-lasting intervals with sustained warm/saline or cold/fresh surface waters. On longer timescales, the 18Ow record shows relatively lighter values at its base (prior to 300BC) and from 150-400 or 500AD, 750-1150AD and 1600-1800AD, suggesting higher salinities. The Mg/Ca SST record generally implies slightly higher temperatures during the same periods, but its structure differs in detail. In particular, climatic anomalies from 150-400/500AD and 750-1150AD are not readily apparent if one solely considers the Mg/Ca record. Subsequently, a pronounced SST drop occurs only after 1400AD, though the onset of an earlier (slight and highly irregular) SST decrease seems to coincide with the initial 18Ow decrease.

These two climate anomalies mentioned above may correspond to the local expression of the Roman (150-400AD) and Medieval (750-1150AD) warm periods, respectively. This would imply an early onset of the subsequent Little Ice Age (after 1150AD in the 18Ow record), but pronounced cooling occurs only after 1400AD and then until 1600AD. The following 1600-1800AD interval seems to be at least as warm and saline as any preceding interval in the record. A second SST and 18Ow decrease comprises also the entire 20th century; accordingly our records seem to show no indication for a post-Little Ice Age recovery.

The general correspondence between the Mg/Ca SST and 18Ow records at Femi Drift is consistent with variable northward advection of warm and saline surface waters during the last 2300 years. On the other hand, divergent patterns (most obvious from 1150-1400AD) may indicate additional local controls on sea surface temperatures such as variable air temperatures. The Feni Drift oxygen isotope record seems to show an intriguing antiphase relationship with a high-resolution record from Vøring Plateau. Due to the combined errors of both calibrated radiocarbon age models, this antiphase relationship cannot be rigorously assessed on multidecadal timescales. On the other hand, it is also apparent for the entire interval 400-800AD and during the 20th century, when age control in both cores is based on radionuclide records. The Vøring Plateau site is located upstream along the flow path of the same warm and saline surface waters, and its isotope record is dominated by temperature fluctuations. Accordingly, this data comparison strongly suggests significant salinity influences on our Feni Drift stable isotope record.

Average foraminiferal weight varied only slightly between 2000 and 0 and between 8000 and 6000 years BP in the Faroe-Shetland Channel and at Orphan Knoll. Between 2000 and 1000 d18O at MD95 2024 was more variable that at HM03-133-25. Mg/Ca varied between 1 and 2.5 mmol/mol in core MD95 2024 and between 1.8 and 3mmol/mol in core HM03-133-25. In both cores, between 1800 and 1600 years BP a broad warm period can be detected, that may possibly be associated with the Roman Warm Period. During the Little Ice Age ( 900 years BP) SST at the Faroe-Shetland channel were cooler. However, cooler intervals can be also be observed 600 years BP and more recently.

Between 2000 and 0 years BP, SST overall seems slightly warmer in the Faroe Shetland channel compared with Orphan Knoll (Labrador Sea), and between 7000 and 6000 years BP a similar trend can be observed. Between 8000 and 7100 years BP (Holocene warm period?) the two sites seem to have similar SST.

Reported by

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