A multi-proxy analysis of late Quaternary Indian monsoon dynamics for the Maldives, Inner Sea

. We present a detailed multi-proxy data record to reveal the late Quaternary changes in marine sedimentation and biogeochemical processes of the upper bathyal Maldives (equatorial Indian Ocean) and how they are related to the benthic ecosystem dynamics. We investigated the sediment core SO-236-052-4 from the central part of the Inner Sea, Maldives, focusing on Fe/Ca and Si/Ca ratios as proxies for terrigenous sediment delivery, as well as Total Organic Carbon (TOC) and Ba/Ca ratios as proxies for marine productivity. Benthic foraminiferal fauna distributions, sortable silt records and stable 15 oxygen and carbon isotope analyses were used for reconstructing the past ecosystem, as well as changes in the intermediate water circulation, bottom water current velocity and oxygenation. This multi-proxy data record shows an enhanced dust supply during the glacial intervals, represented by increased Fe/Ca and Si/Ca ratios, an overall coarsening of the sediment and increasing amount of agglutinated benthic foraminifera. The enhanced dust fluxes can be attributed to higher dust availability in the Asian desert and loess areas and its transport by 20 intensified winter monsoon winds during glacial conditions. These combined effects of wind-induced mixing of surface waters and dust fertilisation during the cold phases resulted in increased surface water productivity and related organic carbon fluxes. Thus, the development of highly diverse benthic foraminiferal faunas and the distribution of certain detritus and suspension feeders were fostered. The difference in the stable carbon isotope between epifaunal and deep infaunal benthic foraminifera reveals 25 intermediate water oxygen concentrations between approximately 40 and 100 µmol kg -1 . The pattern of oxygen changes resembles that from the deep Arabian Sea suggesting an expansion of the Oxygen Minimum Zone (OMZ) from the Arabian Sea into the tropical Indian Ocean, further controlled by the inflow of the Antarctic Intermediate Water (AAIW). The precessional circulation pattern of the bottom water oxygenation is overprinted by glacial-/interglacial changes resulting in a long phase of reduced ventilation during the last glacial period. The latter process is likely linked to the combined effects of 30 generally enhanced oxygen consumption rates during high-productivity phases, reduced AAIW production and restriction of bathyal environments of the Inner Sea of the Maldives during sea-level lowstands. Thus, this multi-proxy record provide a close linkage between the Indian monsoon oscillation, intermediate water circulation, productivity and sea-level changes on orbital time-scale. suborbital Summer monsoon changes are strongly coherent over the precessional band reveal a close but lagged response to maxima in northern insolation Phases of intensified OMZ are related to increased organic matter fluxes and microbial oxygen consumption rates during summer monsoon maxima In contrast, deep vertical mixing and erosion of the OMZ during phases of More recent studied revealed that the intensity of the OMZ and dynamics of deep-sea benthic are paced by the superposition of regional monsoon dynamics and super-regional changes of intermediate and deep-water ventilation

(Solnhofen limestone) together with samples over a 1-year period, precision was better than 0.08 ‰ for δ 18 O and 0.06 ‰ for δ 13 C, respectively.

165
For site SO-236-052 changes in bottom water oxygen concentrations were estimated based on the δ 13 C difference between the epifaunal (C. mabahethi) and deep infaunal (G. affinis s.l.) benthic foraminifera using the function Δδ 13 C = 0.00772 x [O 2 ] + 0.41446, wherein concentrations [O 2 ] < 235 µmol kg -1 are considered as significant (Hoogakker et al., 2015). For comparison, oxygen concentration changes of a deep sea sediment core from an Arabian Sea site (GeoB3004, 1803 m water depth) were taken from Schmiedl and Mackensen (2006). These data based on the difference between the 170 epifaunal Cibicides wuellerstorfi and G. affinis. Further, the δ 13 C gradient between G. ruber (white) and C. mabahethi of core SO-236-052-4 was estimated for assess the sea surface and bottom-water stabile carbon isotope difference and water column mixing.

Radiocarbon dating and compilation of the age model
The Fe/Ca and Si/Ca records reveal generally higher values during glacial and lower values during interglacial periods.
Both Fe/Ca and Si/Ca records are characterized by an abrupt and short-lasting maximum at Termination II (Fig. 4c). The Ba/Ca ratio is comparatively low but shows a similar glacial-interglacial pattern, such as the Fe/Ca and Si/Ca records, with 230 generally higher ratios during glacial periods and lower ratios during interglacial periods, but with additional variability on orbital time scales (Fig. 4d). Inverse patterns are observed for the Sr/Ca record, which follows the inversed δ 18 O curve and shows higher values during interglacial and lower values during glacial periods (Fig. 4d).
The distribution of the most important benthic foraminiferal species, which characterize the three faunal assemblages, are displayed in Figs. 6e-g. The most abundant species include C. mabahethi (maximum relative abundance of ~17 % during MIS 6), the weakly hispid N. proboscidea (maximum of ~16 % at the end of MIS 5) and D. araucana (maximum of ~11 % 250 during the onset of MIS 5). Meroplanktonic benthic foraminifera (genera Cymbaloporetta and Tretomphaloides) occured in elevated numbers during interglacial periods, particularly during MIS 5 (Figs. 5d and 6e).
The foraminiferal fauna at the Inner Sea is dominated by hyaline taxa. In comparison, agglutinated individuals were less abundant, but with increasing relative abundances up to approximately ≥ 20 % of the entire fauna during the glacial periods.

Monsoon variability, dust fluxes and marine productivity
The Fe/Ca and Si/Ca ratios at site SO-236-052 were interpreted as proxies for terrigenous sediment delivery, deposited by aeolian dust fluxes in the Maldives Inner Sea. Local sources of Fe-rich sediments can be excluded since the sediments of the Maldives islands and adjacent deep-water environments are characterized by carbonates comprising reef and lagoon 260 Clim. Past Discuss., doi:10.5194/cp-2017Discuss., doi:10.5194/cp- -54, 2017 Manuscript under review for journal Clim. Past Discussion started: 3 April 2017 c Author(s) 2017. CC-BY 3.0 License. carbonates of the islands and pelagic deep-water carbonates (Betzler et al., 2013b;Reolid et al., 2017). Calcite content is also very high at site SO-236-052 throughout the past 200 ka (Fig. 4b). Studies on modern aerosols of the North Pacific region indicate that most of the oceanic iron input is derived from atmospheric transport after mobilisation from the central Asian deserts and Chinese loess plateau (Duce and Tindale, 1991). Accordingly, the most likely dust sources for the observed Fe in the sediments of the Maldives are the Indian subcontinent and the Asian desert and loess areas (Roberts et al., 2011), 265 although a minor contribution from Africa and Arabia cannot be excluded (Sirocko and Lange, 1991;Chauhan and Shukla, 2016). Coincident to the prevailing wind system, the majority of dust is transported via the NE monsoon, which blows during northern hemisphere winter. Elevated Fe/Ca ratios during the glacial intervals of MIS 6 and MIS 4-2 indicate the combined effects of enhanced glacial dust availability in the source areas and dust transport to the Maldives with generally strengthened NE monsoon winds. Enhanced glacial dust fluxes were also observed in the Arabian Sea associated with a 270 strengthening of northwesterly winds, which blow dust from the Arabian peninsula into the marine areas (deMenocal et al., 1991;Sirocko and Lange, 1991). On a global scale, the generally colder and drier glacial conditions resulted in a two-to fivefold increase of dust fluxes (Maher et al., 2010). Enhanced glacial dust concentrations were also detected in polar ice cores (Ruth et al., 2003;EPICA Community Members, 2004) and a multitude of marine records (deMenocal et al., 1993;Liu et al., 1999;Zhang et al., 1999;Winckler et al., 2008;Maher et al., 2010).

275
The observed response of the winter circulation of the Maldives to glacial conditions is in line with the finding of a general strengthening of the NE Indian monsoon after initiation of the northern hemisphere glaciation (Gupta and Thomas, 2003). However, our Fe/Ca record lacks significant variability on the precessional band, which should be expected if the dust fluxes were directly proportional to the intensity of the winter monsoon (Caley et al., 2011a, b). Therefore the dust record of the Maldives Inner Sea is mainly driven by the generally enhanced dust availability during glacial intervals. As a major dust 280 source, the Chinese loess plateau is strongly influenced by the East Asian Monsoon (EAM). During the late Quaternary, EAM and related vegetation changes are characterized by predominant excentricity cycles associated with the advance and retreat of the boreal ice sheets (Ding et al., 1995;Liu et al., 1999;Sun et al., 2006;Hao et al., 2012). On the Chinese loess plateau the onset of glacial conditions led to an abrupt increase of atmospheric dust loadings (Zhang et al., 2002) suggesting the operation of climate-vegetation feedbacks. Enhanced deposition of dust particles at site SO-236-052 led to a generally 285 coarsening of the glacial sediment (Fig. 4a) and fostered the distribution of agglutinated benthic foraminifera, which reach a relative abundance of up to ~20 % during the last glacial period (Fig. 7e). Most agglutinated foraminifera in core SO-236-052-4 belong to the Textulariida, such as Spiroplectammina sagittula, Textularia calva or Textularia pala. These and related taxa are often associated with relatively coarse-grained substrates and preferentially use siliciclastic grains for test construction (Allen et al., 1999;Murray, 2006;Armynot du Châtelet et al., 2013).

290
The equatorial Indian Ocean is limited in the micronutrient iron (Wiggert et al., 2006;Maher et al., 2010) and therefore, 2016). The TOC and Ba contents of marine sediments are widely used as proxies for organic matter fluxes and surface water productivity (Müller and Suess, 1979;Gingele et al., 1999;McManus et al., 1999;Rühlemann et al., 1999). But the 295 applicability of both proxies in quantitative reconstructions is limited by the specific sedimentological and biogeochemical processes at the sediment-water interface, including the bulk accumulation rate and bottom water oxygenation (Möbius et al., 2011;Schoepfer et al., 2015;Naik et al., 2017). araucana, all with PC scores > 3 in at least one assemblage (Table 2). Microhabitat studies demonstrated that most species of the genera Cibicides and Cicididoides live as suspension feeders on or elevated above the sea floor (Lutze and Thiel, 1989; Linke and Lutze, 1993), therefore we assume a similar microhabitat preference for C. mabahethi. In the Red Sea this 310 species is adapted to relatively high oxygen contents and low organic matter fluxes (Edelman-Furstenberg et al., 2001;Badawi et al., 2005). The cosmopolitan N. proboscidea inhabits an epifaunal to very shallow infaunal microhabitat (Fontanier et al., 2002;Licari et al., 2003) and has been described as detritus feeder from various bathyal and abyssal environments. In the South Atlantic Ocean, N. proboscidea is associated with well-ventilated and oligotrophic conditions (Schmiedl et al., 1997). Whereas this species thrives under moderate to high organic matter fluxes and oxygen-depleted 315 intermediate waters in the Indian Ocean (Murgese and De Deckker, 2007;De and Gupta, 2010) and was used as a proxy for the strength of the SW monsoon (Gupta and Srinivasan, 1992;Gupta and Thomas, 2003;Sarkar and Gupta, 2014). These observations and the high relative abundance of N. proboscidea in core SO-236-052-4 during the last glacial intervals MIS 4-2, as well as the interglacial interval MIS 5 suggest an adaptation to a wide range of trophic conditions and confirms its tolerance to moderate oxygen depletion. Little information is available on the ecology of D. araucana but its flat trochospiral 320 morphology and distribution in the North Atlantic Ocean suggest an epifaunal microhabitat and adaptation to suspended food sources (Corliss and Chen, 1988;Koho et al., 2008). Similar to the closely related D. bertheloti it may prefer mesotrophic and oxic conditions (De, 2010) with a tolerance to moderate oxygen depletion (Edelmann-Furstenberg et al., 2001). The shallow infaunal Hoeglundina elegans is commonly associated with low to moderate organic matter fluxes, fresh phytodetritus and high oxygen contents (Corliss, 1985;Koho et al., 2008).  Table 2). The dominance of D. araucana during MIS 5 and C. mabahethi during MIS 6 and MIS 1 indicate 360 phases of enhanced lateral food supply, which for the interglacial periods (MIS 5, MIS 1) correlate with reconstructed higher current velocities (Fig. 5). The interglacial intervals of SO-236-052-4 (mainly MIS 5 and MIS 7, Fig. 5) contain high abundances of meroplanktonic benthic foraminifera (Cymbaloporetta, Tretomphaloides), which build floating chambers for dispersal (Banner et al., 1985;Alve, 1999). These taxa are commonly found in shelf environments (Milker and Schmiedl, 2012). Their acme during the last interglacial maximum at bathyal depth of the Maldives Inner Sea coincides with almost 365 absence of other displaced species from reef and lagoon environments, such as Elphidium, Amphistegina or Operculina (Parker and Gischler, 2011). This implies a repeated colonization of bathyal environments with meroplanktonic taxa from submerged neritic environments during sea-level highstands and strengthened bottom water velocity.

370
The epibenthic stable carbon isotope record of core SO-236-052-4 lacks a coherent glacial-interglacial pattern but reveals an overall δ 13 C Cm increase of ~0.5 ‰ over the past 200 ka (Fig. 3). Long-term trends of similar magnitude have been recorded from sites bathed by the Antarctic Intermediate Water mass (AAIW) in the southwestern Pacific Ocean (Pahnke and Zahn, 2005;Elmore et al., 2015;Ronge et al., 2015). The general resemblance of the various epibenthic δ 13 C records suggests a significant role of AAIW in ventilation of bathyal environments of the Maldives Inner Sea, which is consistent with the 375 modern oceanographic situation (You, 1998). mabahethi and the deep infaunal G. affinis s.l. The resulting O 2 concentrations never dropped substantially below 45 µmol kg -1 (~1 ml l -1 ) (Fig. 8). Moreover, the oxic to low oxic conditions (O 2 > 1 ml l -1 ) did not seem to pose stress to the benthic  bertheloti to moderately reduced oxygen and increased food levels.

405
Abrupt O 2 drops occur at the end of the last two glaciations suggesting short phases of reduced AAIW advection or increased surface water productivity and related oxygen consumption at depth (Fig. 8). The recorded events correlate with phases of increased Agulhas leakage, which have been linked to a strengthening of the Indian monsoon and Indian Ocean equatorial winds (Peeters et al., 2004). Accordingly, an additional impact of changes in the strength of the Indian Ocean Equatorial Westerlies (IEW) on environmental changes of the Maldives Inner Sea appears likely (Beaufort et al., 1997(Beaufort et al., , 410 2001. However, our new results imply that on orbital time scales changes of the winter monsoon and AAIW advection seem to play the dominant role.

Conclusions
The integrated evaluation of sedimentological, geochemical and micropaleontological proxy records from the Maldives Inner (1) Aeolian dust fluxes were considerably enhanced during glacial intervals (MIS 6 and MIS 4-2) as indicated by increased Fe/Ca and Si/Ca ratios, generally coarsening of the bulk sediment, and increased abundance of agglutinated 420 benthic foraminiferal taxa, which use siliciclastic grains for test formation. The enhanced dust input was linked to phases of Clim. Past Discuss., doi:10.5194/cp-2017-54, 2017 Manuscript under review for journal Clim. Past Discussion started: 3 April 2017 c Author(s) 2017. CC-BY 3.0 License. generally increased atmospheric dust loads and northeast winds, suggesting a close link of Maldives marine environments to the aridity of the central Asian loess areas and the strength of the Indian winter monsoon.
(2) Increased vertical mixing during glacial phases of intensified winter monsoon resulted in enhanced surface water productivity and associated organic carbon fluxes to the sea-floor as indicated by TOC values and composition of the benthic 425 foraminiferal fauna. The Cibicidoides mabahethi (assemblage 2) and Neouvigerina proboscidea (assemblage 3) faunas dominate during MIS 6 and MIS 4-2 respectively, suggesting differences in the amount and quality of the food delivery for the two glacial intervals. The Δδ 13 C Gr-Cm , Ba/Ca and TOC records reveal additional changes on the precessional band, which are inversely correlated to northern hemisphere summer insolation underlining a close link of regional vertical mixing of the water column and marine productivity to the Indian winter monsoon.

430
(3) Glacial-interglacial changes in sea level controlled the downslope transport of sediment from the Maldives islands to the deep-sea environments and influenced the current strength at the benthic boundary layer of the Inner Sea resulting in different grain size and substrates. Hand in hand with sea level changes there was a change in the bottom current regime. The drift deposits recovered by core M74/4-1143 show that highest current intensities occurred during and after the glacial terminations (Fig. 5). Bottom currents in general were stronger during interglacials than during glacials, although core SO-  depth of core SO-236-052-4. Conventional radiocarbon ages were calibrated using the radiocarbon calibration program CALIB (version 7.0.4; Stuiver and Reimer, 1993) and the calibration curve Marine13 (Reimer et al., 2013).