Fuentes y preservación de materia orgánica en sedimentos recientes del estuario del río Yangtze, China

The vertical distributions of bulk and molecular biomarker compositions in the samples from four sediment cores of the Changjiang (Yangtze River) Estuary were determined. The changes in the bulk and molecular compositions with depth suggest that there have been recent changes in the marine autogenic and terrigenous supply. In the site at the boundary of the turbidity maximum (Site 8) and the most southern site (Site 26), the autogenic and allochthonous inputs make almost the same contribution to sedimentary organic matter. In the site close to the river mouth (Site 11), the organic matter mainly comes from the terrestrial input carried by the Changjiang water, whereas, in the most eastern site (Site 17), the organic matter consists of a mixture of recent and ancient Changjiang delta sedimentary residues. Significant downcore fluctuations were observed in the patterns of the bulk and molecular compositions, as well as in several biomarker ratios, which also indicates the different anoxic conditions at different depths of the core besides the source variation. In addition, the distributions of molecular compounds show that the organic matter is a mixture of immature and mature in the sediments of the four cores, which further indicates that the microbial activity is active in anoxic conditions, especially in the surface sediment. The vertical distributions of molecular compounds also show that the autogenic marine organic matter is more easily degraded, and that the molecular compounds evolve from unstable steric configurations to stable ones in the early diagenetic processes.Este artículo analiza la distribución vertical de la composición mayoritaria y de biomarcadores moleculares en muestras de sedimentos de cuatro testigos obtenidos en el estuario del río Yangtze. Los cambios de composición en profundidad sugieren variaciones recientes de los aportes autógenos y terrígenos. En el máximo de turbidez y en sus limites (estación 8) y en la estación más al sur (estación 26), los aportes autógenos y alóctonos tienen casi la misma contribución a la materia orgánica sedimentaria. En la estación próxima a la desembocadura (estación 11), la materia orgánica procede principalmente de los aportes terrestres del río Yangtze, mientras que en la estación situada mas al Este (estación 17), la materia orgánica consiste en una mezcla de aportes recientes del río Yangtze y de residuos sedimentarios de un antiguo delta del mismo río. Fluctuaciones significantes de la composición mayoritaria y molecular, así como de biomarcadores, fueron observadas a lo largo de los testigos, lo cual indica también condiciones de anoxia diferentes a distintas profundidades, aparte de las diferencias en la procedencia de los sedimentos. Además, la distribución de compuestos moleculares muestra que la materia orgánica es una mezcla de sedimentos maduros e inmaduros en los cuatro testigos, lo cual indica que la actividad microbiana es activa en condiciones de anoxia, especialmente en los sedimentos superficiales. La distribución vertical de los compuestos moleculares también muestra que la materia orgánica marina autógena es más fácilmente degradable, y que los compuestos moleculares evolucionan desde configuraciones inestables estéricas a configuraciones estables en las primeras fases de la diagénesis

Geochemistry of Middle Holocene sediments from south Yellow Sea: Implications to provenance and climate change

From a large number of case studies on terrestrial deposits we know that Late Holocene climate fluctuations have an important impact on the terrestrial environments. However, it is hitherto not clear how the marine sediments can be used to shed light on the environment and climate change of the catchment. To provide such insight, we used the major element, trace element and rare earth element (REE) compositions in the southern Yellow Sea (SYS) sediments to reconstruct the changes in weathering and erosion of their source regions. The sediments originate predominantly from the upper crust of East China and are transported into the basin especially by the Yellow River (Huanghe) and to a lesser extent by the Yangtze River (Changjiang). The chemical index of alteration (CIA; 53.7–59.7) suggests low chemical weathering of the source rocks since the Middle Holocene. This is consistent with the relatively cool and arid climate in North China after the Holocene Megathermal. Comparison of element ratios, including Al/Ca, K/Ca, Al/Na, K/Na, Rb/Sr, Li/Ba and the CIA shows that we can use the latter as a robust proxy for climate change. The CIA-based mean annual precipitation and mean annual temperature show a series of climate fluctuations in the catchment. A relatively warm and humid Period I (5.3–2.9 cal. ka BP), a relatively cool and dry Period II (2.9–0.9 cal. ka BP) and an increasingly cool and dry Period III (0.9–0.3 cal. ka BP). These periods can be linked to climate intervals recognized elsewhere

Modulational instability of spin modified quantum magnetosonic waves in Fermi-Dirac-Pauli plasmas

A theoretical and numerical study of the modulational instability of large amplitude quantum magnetosonic waves (QMWs) in a relativistically degenerate plasma is presented. A modified nonlinear Schrödinger equation is derived by using the reductive perturbation method. The modulational instability regions of the QMWs and the corresponding growth rates are significantly affected by the relativistic degeneracy parameter, the Pauli spin magnetization effects, and the equilibrium magnetic field. The dynamics and nonlinear saturation of the modulational instability of QMWs are investigated numerically. It is found that the increase of the relativistic degeneracy parameter can increase the growth rate of the instability, and the system is saturated nonlinearly by the formation of envelope solitary waves. The current investigation may have relevance to astrophysical magnetized compact objects, such as white dwarfs and pulsar magnetospheres

Development of a novel sea surface temperature proxy based on bacterial 3-hydroxy fatty acids

Gram-negative bacterial 3-hydroxy fatty acids (3-OH-FAs) have received recent attention for their potential as palaeoclimate proxies. A novel temperature proxy, the ratio of anteiso to normal C13 3-OH-FA (RAN13), has been proposed for sea surface temperature (SST) reconstruction in the North Pacific Ocean. However, whether this newly proposed temperature proxy is applicable to marginal seas with significant terrigenous input or tropical oligotrophic seas requires further investigation. Here, we analyzed the composition and distribution of 3-OH-FAs and evaluated the possible impact of various environmental parameters (SST, water depth, dissolved oxygen, salinity and nutrient concentration) on their distribution in marine surface sediments from the Bohai Sea (BS) and the South China Sea (SCS). In the BS, the potential source proxy, fractional abundance of anteiso 3-OH-FAs (average 17%), indicates 3-OH-FA geochemical signature are not greatly overprinted by terrigenous inputs. The relative abundance of long-chain 3-OH-FAs (C15-C18) are higher in the SCS (average 41%) compared to those in other seas (average 33% for all marine samples). Massive inputs of terrigenous organic matter to the BS likely result in overestimation of SSTs based on the RAN13 proxy, and limited abundance of anteiso and normal C13 3-OH-FAs in the oligotrophic SCS may increase the uncertainty of the RAN13 estimated SSTs. More importantly, we find that most short-chain 3-OH-FAs are temperature dependent, especially the fractional abundance of i-C12, a-C13, i-C14 and n-C14 with a high determination coefficient (R2 > 0.60). Based on these newly found correlations, we propose a novel proxy: RANs. The RANs index shows a strong linear relationship with SST (R2 = 0.92, p13, especially in tropical samples. Furthermore, the RANs proxy is significantly correlated with TEX86, and RANs-based SSTs are approximate to LDI derived temperature in the SCS, which support the reliability of RANs as a temperature proxy. These findings further suggest 3-OH-FA based proxies have potential for paleo-SST reconstruction, especially at higher and lower ends of the ocean temperature spectrum and even in cases where marginal inputs of terrestrial organic matter and nutrients are high.</p

Isoprenoidal and hydroxylated GDGTs in SPM from Yangtze River Estuary at different water depths

The concentrations of iGDGTs and OH-GDGTs in suspended particulate matter (SPM) from Yangtze River Estuary as well as temperature, oxygen and ammonium contents were analyzed at different water depths in the winter and summer season (February and July 2014)

Leaf wax and Sr‐Nd isotope evidence for high‐latitude dust input to the central South China sea and its implication for fertilization

Recent time-series from sediment traps show abnormally high chlorophyll-a concentrations and primary productivity in the oligotrophic central South China Sea (SCS), especially during wintertime. Here we present new insights from compound-specific hydrogen isotopic analysis of leaf wax n-alkanes and Sr-Nd isotope compositions extracted from four basin-wide surface sediment transects. We find that the deepest surface sediments in the central basin contain the most depleted n-alkane hydrogen isotopes, suggesting inputs from higher latitude soils in northern China. This is supported by the Sr-Nd isotope compositions of the same surface sediments. We propose that aeolian dust is transported by the winter monsoon and might fertilize the phytoplankton bloom in the central SCS. This process may have been enhanced in ancient times when the winter monsoon was stronger, driving both vertical mixing and dust transport to the central basin