Changes in rock magnetic properties of sediments along the Carlsberg Ridge: Inferences on active/extinct vent fields

Abstract

Hydrothermal vents at Mid Ocean Ridges (MOR) are known to leave metalliferous, bacterial, temperature-induced signatures in sediments. Knowledge of active/extinct vents along the Carlsberg Ridge (CR) is sparse; further conventional methods to distinguish relevant areas are tedious/expensive. We show that basic rock magnetic measurements, including magnetic susceptibility and laboratory-induced remanent magnetizations, can help to characterize sediment samples near and away from hydrothermally activity/extinct areas at the CR. Eight spade cores from the Carlsberg ridge (of which three are in the vicinity of the Tianxiu hydrothermal field and one from the ridge flank) collected on board R.V. Sindhu Sadhana during 77th and 83rd cruises are investigated for the present study. Two spade cores consist of top 2 cm and six core lengths ranged between 22 and 38 cm. Coarse fraction and magnetic minerals were separated and studied under a binocular microscope and scanning electron microscope (SEM). Magnetic susceptibility (χlf) of sediments ranges between 1.3 and 37.1 × 10−8 m3 kg−1 with higher values for ridge valley and lower values for ridge flank. Ridge flank sediments are associated with low χlf, Saturation Isothermal Remanent Magnetization (SIRM), Hard Isothermal Remanent Magnetization (HIRM) and high χARM/SIRM, which are taken as base values. Sediments in the vicinity of the Tianxiu hydrothermal vent field are associated with high χlf, SIRM, HIRM, and low Anhysteretic remanent magnetic susceptibility (χARM)/SIRM, coarse Stable Single Domain (SSD) and fine SSD & Mixture grains with magnetite as the dominant mineral. Similar signatures are observed in two more spade cores ∼250 km south of the Tianxiu vent field, suggesting new active/relict site/sites in the vicinity. Signatures like that of flank sediments are also observed in two cores from the ridge valley indicating, the absence of hydrothermal activity in the vicinity. In the marine environment, identifying/locating metalliferous hydrothermal sediment/sites along extensive MOR is expensive and time-consuming; rock magnetic could be a non-destructive method to shortlist the areas for detailed studies.