Pleistocene geochemical stratigraphy of the borehole 1317E (IODP Expedition 307) in Porcupine Seabight, SW of Ireland: applications to palaeoceanography and palaeoclimate of the coral mound development

Abstract

Stable isotopes and element compositions of the fine-grained matrix were measured for IODP Expedition 307 Hole U1317E drilled from the summit of Challenger Mound in Porcupine Seabight, northeast Atlantic, to explore the palaeoceanographic and palaeoclimatic background to development of the deep-water coral mound. The 155 m long mound section was divided into two units by an unconformity at 23.6 mbsf: Unit M1 (2.6-1.7 Ma) and Unit M2 (1.0-0.5 Ma). Results from 519 specimens show a difference in d 13 C value between Unit M1 (À0.6% to À5.0%) and Unit M2 (À1.0% to 1.0%), but such a distinct difference was not seen in d 18 O values (1.0%-2.5%), CaCO 3 content (40-60 wt%), Sr/Ca ratio (2.0-8.0 mmol mol À1 ), and Mg/Ca ratio (10.0-20.0 mmol mol À1 ) through the mound. Positive d 18 O and negative d 13 C shifts at the mound base are consistent with the oceanographic changes in the northeast Atlantic at the beginning of the Quaternary. The positive d 13 C regression in Unit M2 suggests a linkage to the mid Pleistocene intensified glaciation in the Northern Hemisphere. Warm Mediterranean Upper Core Water of Mediterranean Outflow Water, Eastern North Atlantic Water and cold Labrador Sea Water of North Atlantic Deep Water are key oceanographic features that cause spikes and shifts in stable isotope and element composition. However, the stable isotope values of the sediment matrix could not primarily record the glacial-interglacial eustatic/temperature change, but indirectly indicate current regimes of the intermediate oceanic layer where the coral mound grew. Similarly, elemental ratios and CaCO 3 content may not represent the productivity and temperature of surface sea water, respectively, but superpose the fractions from both surface and bottom water. It is concluded that palaeoceanographic change coupled to the Pleistocene glacial/interglacial cycles is a key control on the geochemical stratigraphy of the matrix sediments of the carbonate mound developed in Porcupine Seabight.

Quaternary primary productivity in Porcupine Seabight, NE North Atlantic

“…As shown in Figure 3, the productivity trend from the biogenic MAR and the calcium carbonate P p appears to mirror the trend of δ 13 C values in the two holes [18,46] (Figures 3 and 4). A similar relationship between MAR and δ 13 C values of foraminiferal tests has been reported in the Miocene [51].…”

“…In northeast Atlantic, warm water masses called Eastern North Atlantic (Central) Water (ENAW) and Shelf Edge Current (SEC) had been immerging Porcupine Seabight and shelf southwest to Ireland, planktons may have prospered and resulted in high MAR and P p in both glacial and interglacial periods. During glaciation, the front of continental ice-sheet could have been extended on sea surface over the shelf and slope [18]. Therefore, much nutritive within the ice-sheet could be felt into surface sea water by the melting of glacier, likely resulting in relatively much higher MAR and P p than in interglaciation at the place of mound development.…”

“…The other is that there were other carbonate components formed in bottom seawater. During glaciation, the bottom water could be dominated by warm Mediterranean Upper Core Water (MUCW) [52] of Mediterranean Outflow Water (MOW), and the warm sediments produced relative positive δ 13 C values [18].…”

“…A, images of lithology in gray level; B, age model calibrated by magnetic polar chron [21], and hiatus age intervals with star marks by strontium isotope of coral skeleton [19]; C, Linear sedimentation ratio (LSR); D, content of biogenic opal; E, MAR estimated against on LSR of Unit M1 and M2 respectively; F, MAR estimated against on LSR of each age interval by magnetic polar chron in detail; G, P p estimated from the transfer function by calcium carbonate content; H, calcium carbonate content calibrated; I, primary calcium carbonate content measured in lab; J, carbon isotope values of marl matrix [18]. Note that results from D through H were calibrated by eliminating aragonites of fine coral fragments, pale yellow bands almost represent the duration of glacial dark gray layers, and roughly indicate the immerging of MUCW.…”

“…307 in April and May, 2005, drilled core sections with a high recovery ratio throughout the Challenger Mound at Site U1316, U1317 in the Belgica province, and at U1318 in the outer shelf southwest to Ireland. Subsequent studies on the core sediments after the expedition have focused on chronology and stratigraphy [13][14][15], microbiology [16], carbon-oxygen isotope and element geochemistry [17,18], diagenesis [19], carbonate budget [20], hydrodynamic environments [21,22], but few has been examined on paleoceanography, for instance, on the primary productivity (P p ) in a relationship with the mound development, and the oceanic resume.…”