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Dissertation zugänglich unter
Changes of clay mineral and trace element characteristics of sinking particulate matter during transport into the Deep South China Sea
Veränderungen der Tonmineral- und Spurenelementeigenschaften partikulärer Sinkstoffe während des Transports in die Tiefsee des Südchinesischen Meeres
(2015) Earth and Planetary Science Letters
Dokument 1.pdf (9.309 KB)
Freie Schlagwörter (Deutsch):
Tonminerale , Südchinesisches Meer , partikuläre Sinkstoffe , mariner Transport
Freie Schlagwörter (Englisch):
Clay minerals , South China Sea , particulate matter , marine transport processes
Schmiedl, Gerhard (Prof. Dr.)
Tag der mündlichen Prüfung:
Kurzfassung auf Englisch:
Clay mineral assemblages are used for over-regional weathering reconstruction, which contributes to the pressing topic of climatic change and to the question about human involvement in it. The sediments in and around the South China Sea (SCS) were carefully researched on their clay mineral properties during the last decades. Reconstruction studies are based on the sedimentary record and in order to know where the clay mineral signal came from, its provenance needs to be identified. Provenance analysis often includes several simplified assumptions about sedimentation history, contribution of various sources and alteration of particulate matter composition during transport. So far, most studies dealt with the recent clay mineral assemblages of dischargers and marine deposits, which left most of the marine trajectory of the source-to-sink transport with little observation. It is insufficiently known how particulate matter changes its composition during transport under recent conditions, which shall be the aim of this study. The observation gap was narrowed with clay mineral, geochemistry, grain size and flux data on sinking particulate matter (SPM). The samples were intercepted during the last 25 years from 1987 to 2012 by sediment trap systems, which were moored at eight stations along two trajectories parallel to the main axis of the direction of monsoonal winds across the SCS. SPM was sampled in 14- to 28- day intervals, which was tested on relative clay mineral abundances, grain size distributions and trace element concentrations. In combination with previously studied particle fluxes and main components analyses, clay mineral fluxes and specific element ratios were calculated.
Clay mineralogical and geochemical characteristics of SPM proved to be regionally divers and change down the water column. Lithogenic matter fluxes in the southwestern basin (8.5-68.5 g/m²/y) are much higher than in the northern basin (11.9-18.5 g/m²/y). Clay mineral assemblages are highly smectite enriched near Luzón (76-84%) at the northeastern margin of the SCS, illite and chlorite rich in the northern (62-64%), central (45-56%) and western (60%) basin and nearly proportional in the southwestern basin. Regional diversity and temporal variability of sinking particulate matter are high in the shallow traps and only moderate in the deep traps. Material reaching the shallow traps is mainly transported by monsoonally changing surface current. In the deep basin, a steady circulation homogenizes detrital particulate matter causing the clay mineral assemblages to homogenize. Additionally, smectite increases relatively and absolutely with depth. In the northern and central basin, higher lithogenic matter fluxes in the deep indicate lateral advection (+9-39%). This laterally advected, smectite-rich material from Luzón is suspected to be the major cause of the downward increasing relative smectite abundance (SCS-N: +26%; SCS-NC: +118%; SCS-C +41%). In the southwestern basin, strongly increased lithogenic matter fluxes also indicate lateral advection (+55-457%), but smectite enriches not nearly as much (+13-38%), because no comparably smectite-rich source exists near the southwestern basin.
Our findings on annual clay mineral abundances and fluxes show, that homogenization by deep circulation and general enrichment with smectite mask the regional clay mineral signals of the deep sea sediments. The deeper and farther offshore a sediment trap is deployed, the higher can the relative enrichment of smectite be. Accordingly, this means for the sediment below the traps, that the combination of material from different sources and differential fractionation effects may mask the riverine clay mineral signals. These masking effects augment with distance from the shore and with the variability of currents through the statistical funnel. As the statistical material collection funnel of a sediment trap widens with depth of the respective trap, so does, statistically, the transport distance of the material. With increasing transport distance SPM fractionation and enrichment with smectite get more effective.
In order to verify and specify provenance analysis results obtained by clay mineral assemblages we correlated them to additional trace element data of the same material. We compiled existing and new trace element data of important sources and analyzed the same sinking particulate matter samples on trace elements. Lanthanide patterns and specifically Europium anomalies, LREE/HREE and Zr/Sc ratios are useful to characterize material from Luzón (1.29-1.43; 0.03-0.07; 3.98-5.17), Taiwan (1.06-1.10; 0.9-1.26; 3.7-3.8/12.7-16.3), Hainan (1.43; 1.13; 7.34) and some smaller southern Vietnamese rivers (0.48-0.93; 0.48-1.07; n.d.) (Nha Trang, Dinh, Ninh Hoa and Long Song). The ratios are less useful to identify material from the larger drainage basins (Pearl River, Red River, Mekong), which discharge sediment with a similar, mixed signal from several lithologies. Trace element characteristics at stations SCS-NE, -N and -C confirm the provenance indications of clay mineral assemblages in the northern basin. SPM in the northern basin is mainly composed of material from Taiwan and Luzón. The contribution from South Chinese Rivers, to SPM could not be confirmed by trace elements. At SCS-NE and -N geochemical signals do not vary seasonally, but remain steady throughout the year. SPM reaches the deep central basin in low concentrations, with a diluted, but distinctive trace element signal of Luzón in both SPM and deep sea sediment. In the southwestern basin the major dischargers are not well distinctive, so the provenance indicator fails in this part of the basin. Only few examples of SPM with a strong negative Eu* proved that contribution of the smaller Vietnamese Rivers increases episodically at SCS-S and that the transport pathways are more variable than statistical means indicate.
Episodic contribution variations at SCS-C can only characterize changes of oceanographic conditions due to its remote location. The vertical changes of Lanthanide patterns and element ratios confirm that the cyclonic deep circulation passes between the 1200 m and 1800 m traps. Occasional variations of the Luzón signal intensity in middle and deep traps demonstrate that the circulation appears to shift up- and downwards or vary its lateral reach and intensity irregularly. Whether the negative Eu* and low LREE/HREE values at SCS-W in the Xisha trough originate from Luzón and Taiwan or Hainan Island, could not be resolved satisfactorily. The general Lanthanide distributions at SCS-W remain steady throughout the year, similar as those at the shore-near stations SCS-NE, -N and -SW. Though temporal variations of specific element ratios and relative clay mineral abundances are noteworthy, they cannot have been caused by the monsoon. At both stations, SCS-C and SCS-W, trace element ratios and clay mineral characteristic variations do not correlate, which suggests that clay minerals are not the main carriers of trace elements. At SCS-W the lack of correlation among the elemental ratios prohibits further deductions, as it can be a result of both source mixing and different fractionation during transport. Apparently, clay minerals and other carriers of trace elements are differently affected by transport processes.
Lanthanide distributions measured on SPM intercepted during 1987-1988 at SCS-N, which sampled an El Niño year, strongly resemble that of Taiwan, while during non-ENSO years 2009-2010 and 2011-2012 element distributions clearly show a mixture of SPM from Taiwan with significant contributions from Luzón. Either the availability of Luzón material is strongly reduced during El Niño years or a change in circulation causes that suspension does not reach station SCS-N. In any case, warm/normal/cool variations of the El Niño Southern Oscillation are enough to significantly alter contribution and dominant provenance of sinking particulate matter at this particular station. The periods are too short to leave an impact on the sediment, which resembles the signal of SPM during normal conditions, only in higher concentrations. But for longer phased warm/cold variations, like glaciation events, the impact could be enough to provide variations in the sedimentary record.
Concluding, we find that seasonal variations are limited to lithogenic matter fluxes, while relative clay mineral assemblages display only locally (SCS-SW, SCS-S) significant fluctuations that could be related to monsoonal changes. Whether these fluctuations are related to changes in weathering, discharge rates or monsoonal surface current reversals, however, remains unresolved. At most stations (SCS-NE, SCS-N, SCS-W, SCS-NC, SCS-C, SCS-SC) changes of relative clay mineral abundances cannot be related to seasonal variations of weathering, even at the shallow traps. Despite previous assumptions, the most important mechanism determining SPM’s clay mineral characteristics appears to be deep current activity, which controls both contribution and differential settling. However, trace element characteristics show, that these can vary interannually and episodically. These can indeed reflect climatic warm/cold conditions like the short-scaled ENSO, but it should be expected, that these refer mostly to changes of the marine environment rather than the terrestrial environmental changes.