Christina L. De La Rocha

Silicon isotope fractionation by marine sponges and the reconstruction of the silicon isotope composition of ancient deep water

The silicon isotope composition (d30Si) of biogenic opal provides a view of the silica cycle at times in the past. Reconstructions require the knowledge of silicon isotope fractionation during opal biomineralization. The d30Si of specimens of hexactinellid sponges and demosponges growing in the modern ocean ranged from -1.2‰ to -3.7‰ (n=6), corresponding to the production of opal that has a d30Si value 3.8‰ +/- 0.8‰ more negative than seawater silicic acid and a fractionation factor (a) of 0.9964. This is three times the fractionation observed during opal formation by marine diatoms and terrestrial plants and is the largest fractionation of silicon isotopes observed for any natural process on Earth. The d30Si values of sponge spicules across the Eocene- Oligocene boundary at Ocean Drilling Program Site 689 on Maud Rise range from -1.1‰ to -3.0‰, overlapping the range observed for sponges growing in modern seawater.

The Biological Pump

The biological pump is the set of processes by which inorganic carbon (e.g., carbon dioxide) is fixed into organic matter via photosynthesis and then sequestered away from the atmosphere generally by transport into the deep ocean. This may be accomplished by the passive sinking of particulate organic matter, through the vertical migration of zooplankton, or the downwelling of surface waters rich in dissolved organic matter. In addition to concentrating carbon in the deep sea, the biological pump also significantly affects the distribution of a number of different chemical constituents of ocean water. There is keen interest in being able to predict both the overall capacity and the efficiency of the biological pump in different places and at different times (including in the future). The physical environment, the type of phytoplankton present, the activities of zooplankton, the presence of biominerals and clay minerals, and the structure of the food web all play important roles in determining both the capacity and efficiency of the biological pump on local and regional scales, complicating efforts to portray the biological pump in models.

Palaeoceanography: In hot water

There has long been scepticism about the geochemical evidence that the ancient ocean was markedly warm. A fresh approach bolsters the case for an ocean that, in the distant past, was indeed quite hot.Sands of timeCherts, dense silicaceous rocks containing microcrystalline quartz, are among the best preserved ocean sediments on Earth and their formation dates cover a period from the early Archaean, 3.5 billion years ago, to the present. New data show that the silicon isotopic composition of cherts varies systematically with geological age and is correlated with the oxygen isotope composition. This means that the isotopic composition of cherts is likely to record their formation temperature. Using numerical models of the silicon cycle, the isotopic curve derived from this data provides a unique record of Precambrian seawater temperature variations, with changes from about 70 to 20 °C between 3.5 and 0.8 billion years ago.There has long been scepticism about the geochemical evidence that the ancient ocean was markedly warm. A fresh approach bolsters the case for an ocean that, in the distant past, was indeed quite hot.

In hot water: Palaeoceanography

There has long been scepticism about the geochemical evidence that the ancient ocean was markedly warm. A fresh approach bolsters the case for an ocean that, in the distant past, was indeed quite hot.Sands of timeCherts, dense silicaceous rocks containing microcrystalline quartz, are among the best preserved ocean sediments on Earth and their formation dates cover a period from the early Archaean, 3.5 billion years ago, to the present. New data show that the silicon isotopic composition of cherts varies systematically with geological age and is correlated with the oxygen isotope composition. This means that the isotopic composition of cherts is likely to record their formation temperature. Using numerical models of the silicon cycle, the isotopic curve derived from this data provides a unique record of Precambrian seawater temperature variations, with changes from about 70 to 20 °C between 3.5 and 0.8 billion years ago.There has long been scepticism about the geochemical evidence that the ancient ocean was markedly warm. A fresh approach bolsters the case for an ocean that, in the distant past, was indeed quite hot.