Sources of laminated sediments in the northeastern Arabian Sea off Pakistan and implications for sediment transport mechanisms during the late Holocene

Abstract

Laminated sediments of the continental slope off the Makran coast in the northern Arabian Sea are well-known climate archives and record productivity, as well as supply of material from land. Here, we studied sediment core 275KL off Pakistan in concert with sediment trap, dust and river samples in order to characterize and quantify land-derived material deposited in varves and event layers. We analysed grain sizes, mineral assemblages, bulk components and stable isotopes (δ 13 C, δ 18 O) of carbonates. In winter, enhanced river discharge is the main source of lithogenic matter contributing the major amounts to the total annual sedimentation of the northern Arabian Sea. During the late summer season, lithogenic matter accumulation is slightly enhanced, probably carried along with the south-eastward blowing Levar winds from the Balochistan and the Sistan Basins and the summer monsoon discharge maximum of perennial streams. C/N ratios and stable carbon and oxygen isotopes could not be used to distinguish between organic matter produced on land and in the ocean, whereas stable carbon and oxygen isotope ratios of carbonates suggest that sedimentation of event layers is dominated by direct inputs from land. Catastrophic denudation and storm events occur on average once every 50 years and lead to sedimentation rates that exceed the mean annual sedimentations of 983 g m −2 yr −1 by 6 to 10 times. Nevertheless, due to their rare occurrence, they contributed only 7% to the total sedimentation during the last ca. 5000 years. End-member modelling of grain sizes in accordance with lithogenic matter accumulation rates and event layer frequencies showed that arid conditions prevailed between 4000 and 5000 a BP while more humid conditions commenced around 2000 ka BP in accordance with the Pacific ENSO record.

Sea Level Change Controlled the Sedimentary Processes at the Makran Continental Margin Over the Past 13,000 yr

“…EM2 (average: 7.2%), the intermediate end‐member, has a modal grain size of ~15 μm. Compared with the extremely fine distal eolian dust collected in Karachi (Burdanowitz et al, 2019; Forke et al, 2019), EM2 represents relatively coarse near‐source eolian dust, which is further confirmed by the eolian sand deposits on top of beach ridge at central Makran (Shah‐Hosseini et al, 2018). Since several of the greatest changes in the proportion of EM2 coincide with the peaks in the proportion of EM3, EM2 might also record the high‐intensity episodic events.…”

“…Then, an appropriate amount of sodium hexametaphosphate was added, and the solution was left undisturbed for 24 hr after being washed (more than three rinse cycles) with distilled water. We did not remove biogenic opal from the sediments because the effect of biogenic opal on the grain size analyses was negligible for sediment from the northern Arabian sea (Boll et al, 2014; Forke et al, 2019). The grain size distribution of the terrigenous fraction was measured with a Malvern Mastersizer 2000 at the South China Sea Institute of Oceanology, Chinese Academy of Sciences.…”

Signals of Holocene climate transition amplified by anthropogenic land-use changes in the westerly–Indian monsoon realm

“…Besides a long-term drying trend since the Mid-Holocene, this transition period and the associated change in the precipitation pattern (winter vs. summer precipitation) affected the settlements and agriculture of the Indus civilization (Giosan et al, 2012(Giosan et al, , 2018. The temporal coincidence gives rise to the hypothesis that humans themselves became environmental drivers prior to the onset of the transition period (Fuller et al, 2011;Ruddiman and Thomson, 2001), instead of only being passive victims of climate change. Whether human impacts suffice to influence the global climate is an ongoing debate, but it is very likely that land-cover changes associated with prehistoric cultures modified at least local climate (Dallmeyer and Claussen, 2011).…”

“…These authors have suggested that inefficient early rice farming caused atmospheric CH 4 concentration to increase at around 5 ka BP. Rice agriculture began around 4 ka BP on the Indian subcontinent and near the Indus River region, adding to the atmospheric CH 4 concentration (Fuller et al, 2011). Further, climatic changes during the Mid-Holocene were suggested to have changed the agricultural system in the region to a "multi-cropping" (summer/winter crops) system Wright et al, 2008).…”

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

“…Although, the coccolithophores community in the NE AS varies over the course of the year with higher alkenone fluxes during winter and spring, Böll et al (2014) found that the alkenone-based SST reconstruction is representative for annual mean SST. The lithogenic material at the core site is supplied by fluvial input from the Makran rivers (Forke et al, 2019;Lückge et al, 2002;von Rad et al, 2002;Staubwasser and Sirocko, 2001;Stow et al, 2002) and by aeolian input probably from the Sistan Basin region in the border region of Afghanistan and Iran (Burdanowitz et al, 2019;Forke et al, 2019;Kaskaoutis et al, 2014;Rashki et al, 2012). At present dust from the Sistan region is transported to the NE AS by northerly Levar winds during summer and northeasterly and westerly wind during winter (Hussain et al, 2005;Kaskaoutis et al, 2015;Pease et al, 1998;Rashki et al, 2012;Sirocko et al, 1991;Tindale and Pease, 1999).…”