The terrestrial sediment record indicates that the Earth's climate varied drastically in the Precambrian era (before 550 million years ago), ranging from surface temperatures similar to or higher than today's to global glaciation events. The most continuous record of sea surface temperatures of that time has been derived from variations in oxygen isotope ratios of cherts (siliceous sediments), but the long-term cooling of the oceans inferred from those data has been questioned because the oxygen isotope signature could have been reset through the exchange with hydrothermal fluids after deposition of the sediments. Here we show that the silicon isotopic composition of cherts more than 550 million years old shows systematic variations with age that support the earlier conclusion of long-term ocean cooling and exclude post-depositional exchange as the main source of the isotopic variations. In agreement with other lines of evidence, a model of the silicon cycle in the Precambrian era shows that the observed silicon isotope variations imply seawater temperature changes from about 70 degrees C 3,500 million years ago to about 20 degrees C 800 million years ago.

A palaeotemperature curve for the Precambrian oceans based on silicon isotopes in cherts

Dissolved, particulate, soil and plant samples were collected from the Yangtze River (Changjiang) system in May 1997 and May 2003 to determine the sources and distribution of organic and inorganic matter within the river system. Average dissolved organic carbon (DOC) concentrations within the main stream were 105 μM C in 1997 and 108 μM C in 2003. Particulate organic carbon (POC) ranged from 0.5% to 2.5% of total suspended matter (TSM). Both dissolved inorganic carbon (DIC) and particulate inorganic carbon (PIC) concentrations decreased from upper to lower reaches of the river, within the ranges 1.2–2.7 mM and 0.08–4.3% of TSM, respectively. δ13C and δ15N values for tributaries and the main stream varied from −26.8‰ to −25.1‰ and 2.8‰ to 6.0‰, respectively. A large spatial variation in particulate organic matter (POM) is recorded along the main stream, probably due to the contributions of TSM from major tributaries and POM input from local vegetation sources. The dominance of C-3 plants throughout the entire basin is indicated by δ13C and δ15N values, which range from −28.8‰ to −24.3‰ and from −0.9‰ to 5.5‰, respectively. The δ13C and δ15N values of organic matter within surface soil from alongside tributaries and the main stream vary from −28.9‰ to −24.3‰ and 2.7‰ to 4.5‰, respectively. Although these differences are subtle, there is a slight enrichment of 15N in soils along the main stream. Various approaches, such as C/N and stable isotopes, were used to trace the sources of organic matter within the river. Riverine POM is mostly derived from soil; the contribution from phytoplankton is minor and difficult to trace via the composition of particles. POC flux has decreased from >5 × 106 t yr−1 during the period 1960–1980 to about 2 × 106 t yr−1 in 1997. This trend can be explained by decreasing sediment load within the Yangtze River. The export of TOC from the Yangtze River at the end of the 20th Century is approximately equivalent to that of the Zaire River, less than that of the Amazon River, and higher than that of other large rivers such as the Mississippi. Large amounts of DOC and POC were transported to coastal areas of the East China Sea over a short period during 1998 flood events, containing large amounts of nutrients and pollutants. Such an event could be an important trigger for coastal environmental problems and changes to the health of ecosystems.

Sources and distribution of carbon within the Yangtze River system

Geochemistry of the dissolved load of the Changjiang Basin rivers: Anthropogenic impacts and chemical weathering

https://www.geomar.de/fileadmin/personal/fb1/p-oz/mfrank/Georg_et_al_2006_Chem_Geol.pdf

New sample preparation techniques for the determination of Si isotopic compositions using MC-ICPMS

. Silicon (Si) released as H4SiO4 by weathering of Si-containing solid phases is partly recycled through vegetation before its land-to-rivers transfer. By accumulating in terrestrial plants to a similar extent as some major macronutrients (0.1–10% Si dry weight), Si becomes largely mobile in the soil-plant system. Litter-fall leads to a substantial reactive biogenic silica pool in soil, which contributes to the release of dissolved Si (DSi) in soil solution. Understanding the biogeochemical cycle of silicon in surface environments and the DSi export from soils into rivers is crucial given that the marine primary bio-productivity depends on the availability of H4SiO4 for phytoplankton that requires Si. Continental fluxes of DSi seem to be deeply influenced by climate (temperature and runoff) as well as soil-vegetation systems. Therefore, continental areas can be characterized by various abilities to transfer DSi from soil-plant systems towards rivers. Here we pay special attention to those processes taking place in soil-plant systems and controlling the Si transfer towards rivers. We aim at identifying relevant geochemical tracers of Si pathways within the soil-plant system to obtain a better understanding of the origin of DSi exported towards rivers. In this review, we compare different soil-plant systems (weathering-unlimited and weathering-limited environments) and the variations of the geochemical tracers (Ge/Si ratios and δ30Si) in DSi outputs. We recommend the use of biogeochemical tracers in combination with Si mass-balances and detailed physico-chemical characterization of soil-plant systems to allow better insight in the sources and fate of Si in these biogeochemical systems.

/pdf/tracing-the-origin-of-dissolved-silicon-transferred-from-1zis5q5cky.pdf

Tracing the origin of dissolved silicon transferred from various soil-plant systems towards rivers: a review

Silicon isotope compositions of dissolved silicon and suspended matter in the Yangtze River, China

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Silicon isotope compositions of dissolved silicon and suspended matter in the Yangtze River, China