The Gephyrocapsa sea surface palaeothermometer put to the test: comparison with alkenone and foraminifera proxies off NW Africa

Abstract

Recently a new palaeotemperature proxy was developed, based on the morphology of the coccolithophore genus Gephyrocapsa. Here we apply and validate this new methodology by comparing Gephyrocapsa-derived palaeotemperature estimates with temperature estimates obtained from alkenone ratios, Mg/Ca ratios in foraminiferal calcite, and planktic foraminifera assemblages. We investigated a downcore record off Cape Blanc (GeoB 1048; 20°55′N, 19°43′W), spanning the last 28 000 years, in the vicinity of BOFS core 31K (19°00′N, 20°10′W), for which a detailed multiproxy palaeotemperature record has already been established. Our data show an especially good correspondence between the Gephyrocapsa- and alkenone-derived palaeotemperatures, and thus validate the great potential of the new Gephyrocapsa proxy to reconstruct sea surface water temperatures (SSTs). Both indicate a glacial drop in SST of ∼3°C compared to Holocene values, similar to the Mg/Ca palaeotemperature record, although the latter is consistently ∼2.5°C cooler. Largest discrepancies, both in terms of downcore amplitude changes and same-age sample estimates, are found with available planktic foraminifera transfer function data. This can be due to fundamental differences in the season of production, ecologic preferences and habitats, and/or preservation. On the other hand, it may be a result of differences inherent to the transfer functions used, and their original calibration, i.e. regional vs. global approach or different temperature ranges covered. In addition to the downcore record off Cape Blanc, sediment samples corresponding to the Last Glacial Maximum (LGM) were selected from a dispersed set of eighteen well-dated gravity cores, in order to derive palaeo-SST maps off Northwest Africa during this time-slice. By applying a regional Gephyrocapsa transfer function for the first time, we show that mean SSTs in the Canary Islands region during the LGM were 4–6°C colder than today. We found lowest glacial temperatures (14–15.5°C) offshore Morocco and south of the volcanic Canary Islands, likely due to intensified upwelling related to stronger trade winds, consistent with estimates from CLIMAP and other palaeotemperature reconstructions for the same region. In contrast, for offshore Cape Blanc, today’s centre of perennial upwelling, our temperature estimates for the LGM are significantly warmer (ΔT [today–LGM] 2.5–4°C) than proposed by CLIMAP (ΔT [today–LGM] 6–10°C). However, our results consistently support temperature reconstructions based on alkenones that also indicate rather small temperature changes (ΔT [today–LGM] 2–4°C) in this area. We show that Gephyrocapsa morphotypes may also provide useful qualitative information about relative changes in primary productivity, although no linear relationship is apparent. In summary, our palaeotemperature and palaeoproductivity data support the observation that enhanced glacial productivity appears geographically limited to the northernmost latitudes of the NW African upwelling system, whilst upwelling intensity off Cape Blanc was reduced, despite overall enhanced trade wind intensity.