Sedimentological and ichnological implications of rapid Holocene flooding of a gently sloping mud-dominated incised valley – an example from the Red River (Gulf of Tonkin)

The Gulf of Tonkin coastline migrated at an average rate of ca 60 m year−1 landward during Holocene sea-level rise (20 to 8 ka). Due to a combination of rapid coastline migration and undersupply of sand, neither coastal barriers nor tidal sand bars developed at the mouth of the Red River incised valley. Only a 30 to 80 cm thick sandy interval formed at the base of full-marine deposits. Thus, the river mouth represented a mud-dominated open funnel-shaped estuary during transgression. At the base of the valley fill, a thin fluvial lag deposit marks a period of lowered sea-level when the river did not reach geomorphic equilibrium and was thus prone to erosion. The onset of base-level rise is documented by non-bioturbated to sparsely bioturbated mud that occasionally contains pyrite indicating short-term seawater incursions. Siderite in overlying deposits points to low-salinity estuarine conditions. The open funnel-shaped river mouth favoured upstream incursion of seawater that varied inversely to the seasonal strongly fluctuating discharge: several centimetres to a few tens of centimetres thick intervals showing marine or freshwater dominance alternate, as indicated by bioturbational and physical sedimentary structures, and by the presence of Fe sulphides or siderite, respectively. Recurrent short-term seawater incursions stressed the burrowing fauna. The degree of bioturbation increases upward corresponding to increasing marine influence. The uppermost estuarine sediments are completely bioturbated. The estuarine deposits aggraded on average rapidly, up to several metres kyr−1. Siphonichnidal burrows produced by bivalves, however, document recurrent episodes of enhanced deposition (>0·5 m) and pronounced erosion (<1 m) that are otherwise not recorded. The slope of the incised valley affected the sedimentary facies. In steep valley segments, the marine transgressive surface (equivalent to the onset of full-marine conditions) is accentuated by the Glossifungites ichnofacies, whereas in gently sloped valley segments the marine transgressive surface is gradational and bioturbated. Marine deposits are completely bioturbated

South Asian monsoon history over the past 60 kyr recorded by radiogenic isotopes and clay mineral assemblages in the Andaman Sea

The Late Quaternary variability of the South Asian (or Indian) monsoon has been linked with glacial‐interglacial and millennial scale climatic changes but past rainfall intensity in the river catchments draining into the Andaman Sea remains poorly constrained. Here we use radiogenic Sr, Nd, and Pb isotope compositions of the detrital clay‐size fraction and clay mineral assemblages obtained from sediment core NGHP Site 17 in the Andaman Sea to reconstruct the variability of the South Asian monsoon during the past 60 kyr. Over this time interval εNd values changed little, generally oscillating between −7.3 and −5.3 and the Pb isotope signatures are essentially invariable, which is in contrast to a record located further northeast in the Andaman Sea. This indicates that the source of the detrital clays did not change significantly during the last glacial and deglaciation suggesting the monsoon was spatially stable. The most likely source region is the Irrawaddy river catchment including the Indo‐Burman Ranges with a possible minor contribution from the Andaman Islands. High smectite/(illite + chlorite) ratios (up to 14), as well as low 87Sr/86Sr ratios (0.711) for the Holocene period indicate enhanced chemical weathering and a stronger South Asian monsoon compared to marine oxygen isotope stages 2 and 3. Short, smectite‐poor intervals exhibit markedly radiogenic Sr isotope compositions and document weakening of the South Asian monsoon, which may have been linked to short‐term northern Atlantic climate variability on millennial time scales

Assessing Holocene Evolution of the Parnaiba River Delta using Petrophysical, Radiocarbon Dating, and Sedimentological Analysis

The Parnaíba River Delta, located in northeastern Brazil, is one of the few examples of a deltaic depositional system with its pristine characteristics well preserved. This has placed it the spotlight of scientific research for the most diverse areas. The present work aimed to identify changes in the depositional environment based on stratigraphic and sedimentological analysis. Vibracores were collected in several places such as an old mangrove forest, in active tidal channels and supratidal swamp zones. The study included sediment analysis associated with petrophysics (Gamma Ray Spectrometry) and age dating by the radiocarbon method. In the first phase, the gamma-ray spectral analysis and the photographic record of the cores were carried out, followed by the stratigraphic description (color, texture, presence of organic matter, fossil content, sedimentary structures), as well as the identification and selection of material for dating. Subsequently, the cores were sampled at regular intervals, every 4 cm, for grain size, compositional (organic matter and calcium carbonate quantification), and mineralogical analyses. The results obtained showed mineralogical variations, which indicate changes in sedimentary origin. The gamma-spectrometric data tend to show higher values with the decrease of the grain and are associated with the sedimentological, stratigraphic, and mineralogical data, improving the interpretation of the depositional history. The contents of organic matter and calcium carbonate, in general, presented variations related to the grain size, with fine sediments (fine sand, silt and clay) presenting higher contents of organic matter, while carbonate is related to the presence of shells. Datings showed ages ranging from 4057± 22 BP to recent (1950 AD). The integration and joint analysis of the data allowed a better interpretation of the Late Holocene evolution of the old channels of the Parnaiba river that are currently working as a system of tidal channels

Surface sediment grain-size distribution and sediment transport in the subaqueous Mekong Delta, Vietnam

The sediment transport around the Mekong delta was clarified by using geochemical data and modeling indicating the southwestward transport of re-suspended sediments under the influence of NE monsoon. However, the detail pattern of grain-size distribution of the surface sediments in the subaqueous Mekong River delta has not been well understood. This paper presents characteristics of grain-size parameters of the surface sediments in order to interpret the sediment-transport pattern and the linkage to coastal erosion and deposition off the Mekong River delta. Surface sediments collected in the coastal water of Mekong River delta (as far south as Ca Mau Peninsula) were analyzed to determine the grain-size distribution and sediment transports in subaqueous Mekong delta.  Results indicated that the net sediment transport was southwestward to the Ca Mau peninsula and the sediments were finer from the delta topset downward the delta foreset. In contrast, the sediment particles on the adjacent inner shelf were coarser and are more poorly sorted due to the increase in carbonate shell fragments and the Early Holocene sediments. The sediment grain-size parameters were variable in different sedimentary sub-environments of the subaqueous Mekong delta and controlled by the distance of sediment transport, the hydrodynamic regime of each region and coastal erosion of the delta coast.References Anthony J.E., Brunier G., BessetM., Goichot M., Dusouillez P, Nguyen V.L., 2015. Linking repid erosion of the Mekong River delta to human activities. Scientific Reports, 1-12. Blott S.J and Pye K., 2001. Gradistat: a grain size distrubution and statistics package for the analysis of unconsolidated sediments, Technical Communication. Earth Surface Processes and Landforms, 26, 1237-1248. Dung B.V., Schimanski A., Stattegger K., Phach P.V., Tiep N.T., Hai N.T., Thanh N.T., Phi T.T., 2009. Sandwaves on the Southeast Vietnam Shelf recorded by high resolution seismic profiles: formation and mechanism. Front. Earth Sci. China, 3(1), 9-20. Dung B.V., Stattegger K., Unverricht, D., Phach P.V.,Thanh N.T.,2013. Late Pleistocene-Holocene seismic stratigraphy of the Southeast Vietnam Shelf. Global and Planetary Change 110, 156-169. Hein H., Hein B., Pohlmann T., 2013. Recent dynamics in the region of Mekong water influence. Global and Planetary Change, 110, 183-194. Folk R.L and Ward W.C., 1957. Brazos River bar: a study in the significance of grain size parameters. Journal of Sedimentary Petrology, 27, 3-26.Liu S., Lu P., Liu D., Jin P and Wang W., 2009. Pinpointing source and measuring the lengths of the principal rivers of the world.International Journal of Digital Earth, 2, 80-87. doi:10.1080/17538940902746082.Mothersill J., 1969. A grain size analysis of longshore bars and troughs, Lake Superior, Ontario. Journal of Sedimentary Petrology, 39, 1317-1324.Milliman J.D., and Meade R.H., 1983. World-wide delivery of river sediments to the ocean. Journal of Geology, 91, 1-21. Milliman J.D., Syvistski J.P.M., 1982. Geomorphic/tectonic control of sediment discharge to the oceans: the importance of small mountain rivers. Journal of Geology, 100, 525-544. Pettijohn F.G., Ridge J.D., 1932. A textural variation series of beach sands from Cedar Point, Ohio, Journal of Sedimentary Petrology, 2, 76-88. Ninh P.V (Ed.)., 2003. South China Sea Monograph, Vol II-Meteorology, Marine Hydrology and Hydrodynamics, Hanoi National University Publisher., 565p., Hanoi (in Vietnamese). Nowacki D.J., Ogston A.S., Nittrouer C.A., Fricke A.T, and Tri V.P.D., 2015. Sediment dynamics in the lower Mekong River: Transition from tidal river to estuary. J.Geophys,Res, Oceans, 120, 6363-6383, doi:10.1002/2015JC010754. Kubicki, A., 2008. Large and very large subaqueous delta dunes on the continental shelf off southern Vietnam, South China Sea. Geo-Mar. Lett, 28, 229-238. Doi:10.1007/s00367-008-0103-9. Sibson R., 1981. A brief description of natural neighbor interpolation, in V. Barnett, ed., Interpreting Multivariate Data: John Wiley Sons, 21-36. Stattegger K., Tjallingii R., Saito Y., Michelli M., Thanh N.T., Wetzel A., 2013. Mid to late Holocene sea-level reconstruction of Southeast Vietnam using beachrock and beach-ridge deposits. Global and Planetary Change, 110, 214-222. Szczuciński W., Jagodziński R., Hanebuth T.J.J., Stattegger K., Wetzel A., Mitręga M., Unverricht D., Phach P.V., 2013. Modern sedimentation and sediment dispersal pattern on the continental shelf off the Mekong River delta, South China Sea. Global and Planetary Change, 110, 195-213. Ta T.K.O., Nguyen V.L., Kobayashi I., Tateishi M and Saito Y., 2001. Late Pleistocene-Holocene stratigraphy and delta progradation, the Mekong River delta, South Vietnam. Gondwana Research, 4(4), 779. Ta T.K.O., Nguyen V.L., Tateishi M., Kobayashi I., Saito Y., Nakamura T., 2002a. Sediment facies and Late Holocene progradation of the Mekong River Delta in Bentre Province, southern Vietnam: an example of evolution from a tide-dominated to a tide- and wave-dominated delta. Sedimentary Geology, 152, 313-325. Doi 10.1007/s11707-009-0002-z. Ta T.K.O, Nguyen V.L., Tateishi M., Kobayashi I., Tanabe S., Saito Y., 2002b. Holocene delta evolution and sediment discharge of the Mekong River, southern Vietnam: Quaternary Science Reviews, 21, 1807-1819. Tamura T., Saito Y., Sieng S., Ben B., Kong M., Choup S., Tsukawaki S., 2007. Depositional facies and radiocarbon ages of a drill core from the Mekong River lowland near Phnom Penh, Cambodia: evidence for tidal sedimentation at the time of Holocene maximum flooding. J. Asian Earth Sci., 29, 585-592. Tamura T., Saito Y., Sieng S., Ben B., Kong M., Sim I., Choup S., Akiba F., 2009. Initiation of the Mekong River delta at 8 ka: evidence from the sedimentary succession in the Cambodian lowland. Quaternary Science Review, 28, 327-344. Thanh N.T., Phach P.V., Dung B.V., Statteger K., Anh L D., Anh P.T., 2014. Sedimentary evolution on the inner shelf adjacent to the Camau Pennisular in the lastest Late Pleistocene-Holocene. Journal of Marine Science and Technology (in Vietnamese). Tjallingii R., Stattegger K., Wetzel A., Phach P.V., 2010. Infilling and flooding of the Mekong River incised valley during deglacial sea-level rise. Quaternary Science Reviews, 29, 1432-1444. Xue Z, Liu J.P., DeMaster D., Nguyen V.L., Ta T.K.O., 2010. Late Holocene Evolution of the Mekong Subaqueous Delta, Southern Vietnam. Marine Geology, 269, 46-60. Xue Z., Liu J.P., Ge Q., 2011. Changes in hydrology and sediment delivery of the Mekong River in the last 50 years: connection to damming, monsoon, and ENSO. Earth Surf. Process. Landforms, 36, 296-308. Xue Z., He R., Liu J.P., Warner J.C., 2012. Modeling transport and deposition of the Mekong River sediment. Continental Shelf Research, 37, 66-78. Xue Z., Liu J.P., DeMaster D., Leithold E L., Wan S., Ge Q., Nguyen V.L., Ta T.K.O., 2014. Sedimentary processes on the Mekong subaqueous delta: Clay mineral and geochemical analysis. Journal of Asian Earth Sciences,79, 520-528. Unverricht D., Szczuciński W., Statteger K., Jagodziński R., Le X.T., Kwong L.L.W., 2013. Modern sedimentation and morphology of the subaqueous Mekong Delta, Southern Vietnam. Global and Planetary Change, 110, 223-235. Wolanski E., Nguyen N.H., Spagnol S., 1998. Sediment dynamics during low flow conditions in the Mekong River Estuary, Vietnam. Journal of Coastal Research, 14, 472-482. Wolanski E., Ngoc Huan N., Trong Dao L., Huu Nhan N., Ngoc Thuy N., 1996. Fine sediment dynamics in the Mekong River Estuary, Vietnam. Estuar. Coast. Shelf Sci, 43, 565-582

South Asian monsoon history over the past 60 kyr recorded by radiogenic isotopes and clay mineral assemblages in the Andaman Sea

The Late Quaternary variability of the South Asian (or Indian) monsoon has been linked with glacial-interglacial and millennial scale climatic changes but past rainfall intensity in the river catchments draining into the Andaman Sea remains poorly constrained. Here we use radiogenic Sr, Nd, and Pb isotope compositions of the detrital clay-size fraction and clay mineral assemblages obtained from sediment core NGHP Site 17 in the Andaman Sea to reconstruct the variability of the South Asian monsoon during the past 60 kyr. Over this time interval eNd values changed little, generally oscillating between 27.3 and 25.3 and the Pb isotope signatures are essentially invariable, which is in contrast to a record located further northeast in the Andaman Sea. This indicates that the source of the detrital clays did not change significantly during the last glacial and deglaciation suggesting the monsoon was spatially stable. The most likely source region is the Irrawaddy river catchment including the Indo-Burman Ranges with a possible minor contribution from the Andaman Islands. High smectite/(illite1chlorite) ratios (up to 14), as well as low 87Sr/86Sr ratios (0.711) for the Holocene period indicate enhanced chemical weathering and a stronger South Asian monsoon compared to marine oxygen isotope stages 2 and 3. Short, smectite-poor intervals exhibit markedly radiogenic Sr isotope compositions and document weakening of the South Asian monsoon, which may have been linked to short-term northern Atlantic climate variability on millennial time scales