Glacial–interglacial changes and Holocene variations in Arabian Sea denitrification

Abstract

Abstract. At present, the Arabian Sea has a permanent oxygen minimum zone (OMZ) at water depths between about 100 and 1200 m. Active denitrification in the upper part of the OMZ is recorded by enhanced δ 15 N values in the sediments. Sediment cores show a δ 15 N increase during the middle and late Holocene, which is contrary to the trend in the other two regions of water column denitrification in the eastern tropical North and South Pacific. We calculated composite sea surface temperature (SST) and δ 15 N ratios in time slices of 1000 years of the last 25 kyr to better understand the reasons for the establishment of the Arabian Sea OMZ and its response to changes in the Asian monsoon system. Low δ 15 N values of 4-7 ‰ during the last glacial maximum (LGM) and stadials (Younger Dryas and Heinrich events) suggest that denitrification was inactive or weak during Pleistocene cold phases, while warm interstadials (ISs) had elevated δ 15 N. Fast changes in upwelling intensities and OMZ ventilation from the Antarctic were responsible for these strong millennial-scale variations during the glacial. During the entire Holocene δ 15 N values > 6 ‰ indicate a relatively stable OMZ with enhanced denitrification. The OMZ develops parallel to the strengthening of the SW monsoon and monsoonal upwelling after the LGM. Despite the relatively stable climatic conditions of the Holocene, the δ 15 N records show regionally different trends in the Arabian Sea. In the upwelling areas in the western part of the basin, δ 15 N values are lower during the mid-Holocene (4.2-8.2 ka BP) compared to the late Holocene (< 4.2 ka BP) due to stronger ventilation of the OMZ during the period of the most intense southwest monsoonal upwelling. In contrast, δ 15 N values in the northern and eastern Arabian Sea rose during the last 8 kyr. The displacement of the core of the OMZ from the region of maximum productivity in the western Arabian Sea to its present position in the northeast was established during the middle and late Holocene. This was probably caused by (i) reduced ventilation due to a longer residence time of OMZ waters and (ii) augmented by rising oxygen consumption due to enhanced northeast-monsoon-driven biological productivity. This concurs with the results of the Kiel Climate Model, which show an increase in OMZ volume during the last 9 kyr related to the increasing age of the OMZ water mass.

Climate and marine biogeochemistry during the Holocene from transient model simulations

“…An earlier version of the accelerated experiment BGC-HOLx10 was compared to sediment core based estimates of Holocene OMZ evolution in the Arabian Sea. Based on δ 15 N records, Gaye et al (2017) find that the AS OMZ has intensified since the last LGM, and that most of this increase occurred throughout the Holocene (their Fig. 6), which is in agreement with the model results presented here.…”

“…For large POC, the settling velocity increases further with depth. In the model version employed here, the simulation of the settling velocity of large detritus is formulated, allowing for the ballast effect of calcite and opal shells according to Gehlen et al (2006). The settling velocity of small POC remains constant at 2 m d −1 .…”

“…In BGC-CTL average water mass age and OMZ volume are much less variable than for the transient experiments. Note that the results for the Arabian Sea in Gaye et al (2017) are from an earlier accelerated experiment, started at year 1500 of KCM-CTL. The earlier results are quite similar but not identical to BGC-HOLx10.…”

Holocene climatic changes in the Westerly-Indian Monsoon realm and its anthropogenic impact

“…Waning glacial conditions in the region during the Early Holocene resulted in consistently lower SSTs until ca. 8.7 ka BP in the NE AS (Böll et al, 2015;Gaye et al, 2018;Giosan et al, 2018) ( Figure 2). Between 8.7 and 8.1 ka BP SST increased from 26.0°C to 27.2°C and was accompanied by rising LIT MAR around 8.4 to 8.3 ka BP.…”

Preface: Ernst Maier-Reimer and his way of modelling the ocean

“…The issue includes papers on additions of new tracer cycles to ocean biogeochemical models (Archer and Blum, 2018;Pätsch et al, 2018;van Hulten et al, 2017), on important questions related to ocean biogeochemical processes and their impacts on tracer distributions (Aumont et al, 2017;Rixen et al, 2019), and on tracer transport within the ocean (Ayache et al, 2017;Racapé et al, 2018;Rae and Broecker, 2018). Further papers address the topics of climate dynamics and impacts (Gaye et al, 2018;Heinze et al, 2018;Schwinger et al, 2017;Segschneider et al, 2018) as well as a critical appraisal of geoengineering (Lauvset et al, 2017). The collection is rounded off with a paper on model complexity (Kriest, 2017) and an outlook paper (Hense et al, 2017).…”