Recent Foraminifers and Stable Isotopes Records on a Bathymetric Transect off Portugal (Cruise JC089) and implications for the Palaeoxygenation proxy

The oceanographic cruise 89 (RRS James Cook) sailed in 2013 off the Iberian margin in support of an IODP proposal centred on IODP Site 1385. JC089 collected a range of hydrographic data and recovered a set of short sediment cores. We focus on 11 of the later, sampling the hydrography of the last c.400 years along a bathymetric gradient (600-4600 m). The stable isotopes (δ¹⁸O & δ¹³C) for: 8 common benthic foraminifer species with varied habitat preferences, the sediment pore-water and the bottom water above the sites were measured. The geochemical data is compared to various sedimentary and micropalaeontological data. The later comprises abundances of the main benthic foraminifera species >212μm, checking for living position of the endo-fauna in Rose-Bengal stained samples and for the abundances of phytodetritus-loving species E.exigua in the >90μm for all the 0-1cm samples. The study of the planktonic foraminifer assemblages along a gradient stretching 170 km offshore confirms the major influence of the upwelling to the East. Except for the epi-benthic species C.wuellestorfi, which records the bottom water δ¹³C at equilibrium, all other species failed to record the δ¹³C of the (pore) water at the depth of their living-position. We find that G.affinis could record the δ¹³C_DIC near equilibrium with the pore-water at a depth of c.-1cm; therefore above its living population peak. This could be explained by vertical migrations through the sediment column at sites where the supply of organic matter is pulsed. The later assumption seems supported by a reverse correlation between high relative abundances of E.exigua and that of the planktonic upwelling indicator species G.bulloides under productivity pulses corresponding to higher Δδ¹³C(epi-G.affinis).

The Δδ¹³C varies from 1.7 to 4.9‰ (n=6) across a decreasing but increasingly pulsed surface productivity gradient further away from the coast. Across this range, G.affinis is observed living at increasing depths in the sediment but always peaks in oxic sediments. The absence of G.affinis from water deeper than 3100 meters prevents Δδ¹³C estimates at deeper water depths. For 6 of the 11 sites where G.affinis was present C.wuellestorfi occurred only twice. The δ¹³C for H.elegans and C.mundulus adjusted by -1.08 and +0.25‰ respectively (this study) were used instead for the shallower sites. Off the Iberian Margin the style of seasonally fluctuating food supply could be the main factor on Δδ¹³C. The implication on future and long-ranging IODP-based palaeoclimatic studies is that the Δδ¹³C could be used to estimate the type of productivity regime back in time. In the one hand the sites mostly influenced by the main upwelling cell exhibit Δδ¹³C < 3‰ & correspond to less than 10% of the time spent in an oligotrophic setting below 0.2mg (chla)/m³. In the other hand Δδ¹³C >3‰ trace offshore rare productive surface filaments in an environment otherwise corresponding to c.90% of the time under oligotrophic surface water. The absence G.affinis (for the range of depths studied) could indicate a record sitting outside either of these productive systems' influence.