Radiocarbon dating on cold-water corals and planktonic foraminifera, grain size analysis, stable oxygen isotopes, and XRF data of sediment cores from the Alboran Sea

Cold-water corals are common along the Moroccan continental margin off Melilla in the Alboran Sea (western Mediterranean Sea), where they colonise and largely cover mound and ridge structures. Radiocarbon ages of the reef-forming coral species Lophelia pertusa and Madrepora oculata sampled from those structures, reveal that they were prolific in this area during the last glacial-interglacial transition with pronounced growth periods covering the Bølling-Allerød interstadial (13.5-12.8 ka BP) and the Early Holocene (11.3-9.8 ka BP). Their proliferation during these periods is expressed in vertical accumulation rates for an individual coral ridge of 266-419 cm ka**-1 that consists of coral fragments embedded in a hemipelagic sediment matrix. Following a period of coral absence, as noted in the records, cold-water corals re-colonised the area during the Mid-Holocene (5.4 ka BP) and underwater photographs indicate that corals currently thrive there. It appears that periods of sustained cold-water coral growth in the Melilla Coral Province were closely linked to phases of high marine productivity. The increased productivity was related to the deglacial formation of the most recent organic rich layer in the western Mediterranean Sea and to the development of modern circulation patterns in the Alboran Sea.

The effects of thermal and high-CO2 stresses on the metabolism and surrounding microenvironment of the coral Galaxea fascicularis

The effects of elevated temperature and high pCO2 on the metabolism of Galaxea fascicularis were studied with oxygen and pH microsensors. Photosynthesis and respiration rates were evaluated from the oxygen fluxes from and to the coral polyps. High-temperature alone lowered both photosynthetic and respiration rates. High pCO2 alone did not significantly affect either photosynthesis or respiration rates. Under a combination of high-temperature and high-CO2, the photosynthetic rate increased to values close to those of the controls. The same pH in the diffusion boundary layer was observed under light in both (400 and 750 ppm) CO2 treatments, but decreased significantly in the dark as a result of increased CO2. The ATP contents decreased with increasing temperature. The effects of temperature on the metabolism of corals were stronger than the effects of increased CO2. The effects of acidification were minimal without combined temperature stress. However, acidification combined with higher temperature may affect coral metabolism due to the amplification of diel variations in the microenvironment surrounding the coral and the decrease in ATP contents.

Biogeochemical parameters (oxygen, nitrate, phosphate, TOC and strontium) at three sites from the Clarion-Clipperton Fracture Zone

The Clarion-Clipperton Fracture Zone (CCFZ) in the Pacific Ocean is characterized by organic carbon-starved sediments and meter-scale oxygen penetration into the sediment. Furthermore, numerous seamounts occur throughout its deep-sea plain, which may serve as conduits for low-temperature hydrothermal circulation of seawater through the oceanic crust. Recent studies in deep-sea environments of the Pacific and Atlantic Oceans have suggested and presented evidence of an exchange of dissolved constituents between the seawater flowing in the basaltic crust and the pore water of the overlying sediments. Through high-resolution pore-water oxygen and nutrient measurements, we examined fluxes and geochemical interactions between the seamount basaltic basement and pore waters of the overlying sediments at three sites located on a radial transect from the foot of Teddy Bare, a small seamount in the CCFZ. At three sites, located 1000, 700 and 400 m away from the foot of the seamount, we found that oxygen concentrations initially decrease with sediment depth but start to increase at depths of 3 and 7 m towards the basaltic basement. NO32- concentrations mirror the oxygen concentration profiles, as they increase with sediment depth but decrease towards the basement. We performed transport reaction modeling and determined at one site the 87Sr/86Sr ratio of the pore water and the bottom water overlying the sediments, which indicated that the 87Sr/86Sr ratio of the pore water at the bottom of the sediment column is similar to the seawater Transport-reaction modeling revealed that (1) the diffusive flux of oxygen from the basaltic basement outpaces the oxygen consumption through organic matter oxidation and nitrification in the basal sediments and (2) the nutrient exchange between the sediment and the underlying basaltic crust occurs at orders-of-magnitude lower rates than between the upper sediment and the overlying bottom water. Our results suggest an upward diffusion of oxygen from seawater circulating within the seamount crust into the overlying basal sediments. The oxygen profiles presented here represent the first of their kind ever measured in the Pacific Ocean, as they indicate an upward flux of molecular oxygen from a basaltic aquifer, something that has so far only been documented - at one other location worldwide - the North Pond site in the Atlantic Ocean. We show that the diffusion of oxygen from the seamount basaltic basement into the overlying pore waters affects the preservation of organic compounds and helps to maintain a completely oxygenated sedimentary column at all 3 sites near the seamount.