Paleoclimatic and diagenetic history of the late quaternary sediments in a core from the Southeastern Arabian Sea: Geochemical and magnetic signals

Abstract

The geochemical and rock-magnetic investigations were carried out on a sediment core collected from the SE Arabian Sea at 1420 m depth in oxygenated waters below the present day oxygen minimum zone. The top 250 cm sediments of the core represent the last 35 kaBP. The δ 18O values of Globigerinoides ruber are heaviest during the Last Glacial Maximum (LGM) and appear unaffected by low-saline waters transported from the Bay of Bengal by stronger northeast monsoon and West Indian coastal current. The signatures of Bφlling-Allerφd and Younger Dryas events are distinct in the records of magnetic susceptibility, organic carbon (OC) and δ 18O. Glacial sediments show higher OC, CaCO 3 , Ba, Mo, U and Cd, while the early to late Holocene sediments show increasing concentrations of OC, CaCO 3 , Ba, Cu, Ni and Zn and decreasing concentrations of Mo, U and Cd. Productivity induced lowoxygenated bottom waters and reducing sedimentary conditions during glaciation and, productivity and oxygenated bottom waters in Holocene are responsible for their variation. The core exhibits different stages of diagenesis at different sediment intervals. The occurrence of fine-grained, low-coercivity, ferrimagnetic mineral during glacial period is indicative of its formation in organic-rich, anoxic sediments, that may be analogous to the diagenetic magnetic enhancement known in sapropels of the Mediterranean Sea and Japan Sea. The glacial sediments exhibiting reductive diagenesis with anoxic sedimentary environment in this core correspond to reductive diagenesis and intermittent bioturbation (oxygenation) reported in another core in the vicinity. This suggests that the low-oxygenated bottom water conditions during glacial times should not be generalized, but are influenced locally by productivity, sedimentation rates and sediment reworking.

Spatiotemporal variations of nitrogen isotopic records in the Arabian Sea

“…Regardless of the declining summer monsoon strength since 5500 ka (Hong et al, 2003), primary productivity in the northern AS seemed to have increased. Similar to the patterns observed for TOC and TN in this study, productivity indicators (TOC and Ba / Al ratios), reported by Rao et al (2010) in the core SK148/4 located near our SK177/11, also increased gradually since the Holocene. Incomplete nitrate consumption can hardly explain the decreasing pattern for all three cores in the southern AS, where upwelling intensity is much less relative to that in the north.…”

Evidence of Poor Bottom Water Ventilation during LGM in the Equatorial Indian Ocean

“…Thus, during LGM, anoxic conditions persisted as indicated by redox sensitive elements, while no decipherable reducing conditions occurred during the early Holocene (11-8 ka years BP). A similar observation in SE Arabian Sea suggests high productivity induced low oxygenated bottom water conditions during LGM (Rao et al, 2010). In comparison to the present study, it has been reported that the sediments from the equatorial Arabian Sea (Pailler et al, 2002) and SE Arabian Sea (Pattan and Pearce, 2009) never witnessed complete anoxic bottom water conditions, instead a fluctuation between oxic and suboxic conditions has been observed during LGM.…”

“…An average sedimentation rate of ∼8 cm.ka −1 was observed for the studied sediment core (Figure 2). The late-Quaternary period has been broadly divided into Last Glacial Maxima (LGM: 22.5-17.5 ka years BP), Deglacial Period (DP: 17.5-11 ka years BP), and Holocene Period (HP: 11-Present) (Rao et al, 2010).…”

“…During LGM, reduced proportion of NADW and increased proportion of AABW near the equatorial Indian Ocean has been inferred on the basis of Nd-isotopic composition of ferromagnese oxides and δ 13 C of foraminifera implying a major trigger in the development of oxygen deficient bottom conditions (Piotrowski et al, 2009). There have been substantial studies indicating the development and persistence of oxygen deficient conditions in the Northern Indian Ocean that are related to either productivity or change in bottom water conditions (Naqvi, 1987;Pailler et al, 2002;Rao et al, 2010). However, there remains unaddressed issues on the simultaneous signatures of productivity and bottom water anoxia on redox sensitive elements.…”

Late Glacial to Holocene Indian Summer Monsoon Variability Based upon Sediment Records Taken from the Bay of Bengal

“…This seasonal low atmospheric pressure at the Tibetan Plateau relative to higher pressure over the cooler Indian Ocean sets the stage for the cyclonic summer-monsoon wind patterns (Webster et al 1998;Rashid et al 2007). The cyclonic wind intensifies upwelling at the Somali margin, bringing up cold, nutrient rich waters that stimulate high productivity (Honjo et al 1999;Heinz and Hemleben 2006;Rao et al 2010). Evaporation from the tropical Indian Ocean adds moisture and latent heat intensifying the ISM (Shankar et al 2002).…”