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Controls on organic matter distribution in oxygen minimum zone sediments from the continental slope off western India
Journal of Marine Systems ( IF 2.8 ) Pub Date : 2020-07-01 , DOI: 10.1016/j.jmarsys.2018.09.003
Siby Kurian , Pratima M. Kessarkar , V. Purnachandra Rao , K. Reshma , Amit Sarkar , J.N. Pattan , S.W.A. Naqvi

Abstract The causes of organic-rich sediments in contact with the perennial oxygen minimum zone (OMZ) on the continental slope off western India have been debated extensively, largely in relation to two main controlling factors: high productivity in the water column and enhanced preservation due to oxygen deficiency in bottom waters. In order to better understand the causes for organic-richness in sediments, three sediment cores recovered from the upper OMZ (UOMZ) at 500 m, middle OMZ (MOMZ) at 812 m and lower OMZ (LOMZ) at 1168 m water depths from the continental slope off Ratnagiri (west coast of India) were investigated for their organic matter (OM) and their stable isotopic (δ13C and δ15N), geochemical and rock magnetic parameters. The bottom water dissolved oxygen (DO) concentrations were 0.4 μM, 5.8 μM and 25 μM at UOMZ, MOMZ and LOMZ core locations respectively. Maximum age of the sediments recorded in these cores was 4500 yrs BP. Sedimentation rates in the upper sections of the UOMZ and MOMZ cores were lower (5 to 7 cm/kyr) than that in the LOMZ core (20 cm/kyr). Organic carbon (Corg) content was highest (upto 7.9%) in the UOMZ core than in other cores (upto 4.9%). Enriched δ13Corg values (−18.6 to −16.8‰) in the MOMZ core were indicative of reworked organic matter in the lower section. The δ15N values ranged from 4 to 7.4‰, however, with lower values for the reworked sediments at MOMZ site. The parameters representing productivity and redox conditions increased towards core top in all the cores, whereas terrigenous parameters decreased towards core top in the MOMZ and LOMZ cores. Despite the MOMZ core was always in OMZ in the lower section, very low Corg content coinciding with high terrigenous and reworked OM were deposited in the sediments prior to 1700 years BP. Rock magnetic properties indicate that the degree of diagenetic conditions varied in different sections of the cores and also among the cores within OMZ. Authigenic magnetite and authigenic uranium occurred in the lower sections of UOMZ and LOMZ cores and, authigenic magnetite in the upper sections of MOMZ core. These results suggest that productivity played an important role in organic-richness of the sediments and bottom water oxygen concentration has little effect on the preservation of OM. Besides, sedimentation rates, reworked organic matter, and topography at the core site indeed contribute on the distribution of organic matter in the Late Holocene sediments.

中文翻译:

印度西部大陆坡最低氧区沉积物中有机质分布的控制

摘要 与印度西部大陆坡常年最低氧区 (OMZ) 接触的富含有机质沉积物的成因已引起广泛争论,主要与两个主要控制因素有关:水体的高生产力和由于底水缺氧。为了更好地了解沉积物中有机质丰富的成因,从 500 m 的上 OMZ (UOMZ)、812 m 的中 OMZ (MOMZ) 和 1168 m 水深的下 OMZ (LOMZ) 中回收了三个沉积岩心。研究了 Ratnagiri(印度西海岸)外大陆坡的有机质(OM)和稳定同位素(δ13C 和 δ15N)、地球化学和岩石磁参数。UOMZ 的底部水溶解氧 (DO) 浓度分别为 0.4 μM、5.8 μM 和 25 μM,分别是 MOMZ 和 LOMZ 核心位置。这些岩心中记录的沉积物的最大年龄为 4500 年 BP。UOMZ 和 MOMZ 岩心上部的沉降速率(5 至 7 cm/kyr)低于 LOMZ 岩心(20 cm/kyr)。UOMZ 岩心中的有机碳 (Corg) 含量最高(高达 7.9%),高于其他岩心(高达 4.9%)。MOMZ 核心中丰富的 δ13Corg 值(-18.6 至 -16.8‰)表明下部有机质发生了再加工。δ15N 值范围为 4 到 7.4‰,然而,MOMZ 站点的返工沉积物的值较低。代表生产力和氧化还原条件的参数在所有核心中都向核心顶部增加,而陆源参数在 MOMZ 和 LOMZ 核心中向核心顶部减少。尽管 MOMZ 核心始终位于下部的 OMZ,在 BP 1700 年之前,极低的 Corg 含量与高陆源和再加工的 OM 沉积在沉积物中。岩石磁性表明,在岩心的不同部分以及OMZ内的岩心之间,成岩条件的程度不同。自生磁铁矿和自生铀出现在 UOMZ 和 LOMZ 核心下部,自生磁铁矿出现在 MOMZ 核心上部。这些结果表明生产力对沉积物的有机丰富度起重要作用,底水氧浓度对有机质的保存影响不大。此外,沉积速率、再加工有机质和核心部位的地形确实对晚全新世沉积物中有机质的分布有影响。
更新日期:2020-07-01
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