Contourites of the Gulf of Cadiz: geochemical and paleoclimate proxies

Citation

Smillie Z & Stow D (2017) Contourites of the Gulf of Cadiz: geochemical and paleoclimate proxies. The Royal Society's Daphne Jackson Research Conference, London, 02.11.2017-02.11.2017.

Abstract
Major currents beneath the ocean surface form the hidden part of a global conveyor belt of ocean circulation that transfers heat and helps buffer Earth's climate. Marked climate changes affect the nature and strength of these bottom currents and are recorded in the sediments they deposit and across which they flow. Decoding the sediment record for the influence of bottom currents allows us to better understand Earth's climate history. Critical ocean gateways, such as the Gibraltar Gateway, are significant for the circulation of the ocean currents and can affect the redistribution of salinity and temperature in the ocean. Recently, scientists worked on an international scientific drilling expedition (IODP Expedition 339) within the Gulf of Cádiz, collecting core samples of sediments to obtain a detailed record of the history of the currents. The majority of these sediments are known as "contourites", so named as they were formed by the action of bottom currents that flow parallel to the slope of the ocean basin and follow its contours. The contourites display significant features of sediment cyclicity with gradational changes from fine-grained sediments toward coarser sediments then followed upwards again by fine-grained sediments. Such changes reflect variation pattern in sedimentation process and current strength. The coarser the sediments involved in the contourite deposit, the stronger the currents, whilst the thicker the deposit, the longer the current duration. Also, strong currents can cause long periods of non-deposition/erosion, resulting in breaks in the sedimentary record as detected from the faunal records. Mineralogical and geochemical investigations are used to document such changes in the current, hence climatic changes. Transmitted light microscopy is used to investigate the distribution of clastic versus biogenic components as well various textural patterns of the sediments. Isotope-ratio mass spectrometry (CF-IRMS) of the organic carbon-13, inorganic carbon-13 and oxygen-18 of the contourites are applied to determine the nature of controls on the millennial scale sediment cycles. They also used to decipher diagenetic (post-depositional) footprints within the rocks. i.e. dolomite formation along the unconformity surfaces. Variation in contourite characteristics is observed at all scales of observation, from the sequence stratigraphic context (100s of metres), to seismic cycles (10s of metres), and to sediment facies cycles depicted in the standard facies model (metres). Controls on such cyclic variation include sea-level, climate, ocean resonance and current instabilities.

Notes
Daphne Jackson Research Conference ; Conference date: 02-11-2017 Through 02-11-2017