User:Boyd7854/sandbox

Climate and sea level

Traditionally it was believed that the Silurian was a relatively calm but short period best noted for the evolution and spreading of plants on Earth’s surface. The Silurian is in a transition zone between the Late Ordovician ice-house climate and the Devonian greenhouse. The distribution of land was primarily south of the equator and at low latitudes. There was land at the south pole and a large Arctic ocean. Sea levels were high during the Silurian. Combine the high sea levels with low continental relief led to large shallow epicontinental seas. Reefs were widely distributed except during the Malvinokaffric. Any extinction events comparable to the Ordovician or Devonian periods were unknown. Though in the past two decades, thoughts about the Silurian being calm have dramatically changed. Through investigations of carbon and oxygen isotopes show a very volatile ocean-atmosphere system. These investigations also led to the discovery of for major positive stable carbon excursions in the Silurian. With the Early Wenlock, Late Wenlock, Late Ludlow and Silurian-Devorian boundary being these four. The changes in the global carbon cycle were far more frequent during the Silurian than any other time in the Phanerozoic. . Tracking sea-level changes through geochemical proxies have largely developed over the past decade. 18O is left in the ocean when 16O is taken during the formation of continental ice sheets. 18O is then taken and incorporated into the structure of calcitic shells as well as conodonts.

Isotopes

Here is a little insight into what each isotope means in terms of climate/weathering. 13C is a proxy for productivity. An excursion towards more positive values in 13COrganic translates to increased organic-carbon burial. This is usually seen as an extinction event. Sr(Strontium) level variations are possibly caused by rates of hydrothermal interaction with basaltic rocks, continental weathering, and/or changes in the 87Sr/86Sr ratio of the continental source material being weathered, for marine Sr. There would be a higher 87Sr/86Sr ratio in rivers from weathering of continental, non-volcanic silicate rocks. Tectonic uplift (continent-continent) may cause the input of riverine Sr.

Glaciations

On Gondwana there are three known glaciations. The first one occurred in the early Aeronian. It was found through dating diamictites overlaying rocks containing Coronograptus gregarius (LAPWORTH) and some chitinozoans, including Conochitina cf. iklaensis NESTOR [Caputo 1998].

The second one occurred in the latest Aeronian- early Telychian and was dated thought the association of chitinozoans in shale’s occurring lateral to the tillites.

The third and final one occurred during the latest Telychian – earliest Sheinwoodian. Glacial rocks were dated via chitinozoans.

All these glacial events were accompanied by sea level drops and extinction events. The main glaciation (Sheinwoodian (also called Ireviken)) has been traced globally. The Aeronian events only have been documented in the Eastern Baltic States.

Atmospheric Conditions and Generalities

The level of atmospheric oxygen is dependent on several factors. These include the rate of photosynthesis, the marine phosphorous cycle, weathering of organic matter and pyrite in sedimentary rocks, reduction and removal of DIC (dissolved inorganic carbon) from sea water, and geochemical reactions of mid-ocean-ridge basalts ([Berner, 2001], [Berner, 2006b] and [Algeo and Ingall, 2007]). Additionally CO2 content of the ocean/atmosphere system depends on various factors, e.g., rate of photosynthesis, weathering of silicates and carbonates, volcanism, nutrient cycling, burial/oxidation of organic matter, and deposition of carbonate rocks ( [Goddéris et al., 2001], [Royer et al., 2001] and [Berner, 2003]). Atmospheric Oxygen remained within 5% of the current O2 levels today according to (Berner 2006) and (Algeo and Ingall 2007). Atmospheric CO2 Remained fairly steady at the beginning of the Silurian at around 4000-4500 ppm then sharply dropped at the end of the Silurian to around 2000 ppm.

Perturbations

At the end of the each of the four stages in the Silurian there's evidence higher positive δ13C values. These are called the early Sheinwoodian A-period, late Homerian A-period, Late Gorstian A-period and the late Ludfordian A-period. All these are associated with biotic extinction events. Other similar perturbations occurred in the Ordovician-Silurian boundary and the Silurian-Devonian boundary . The Ireviken event, Mulde event and Lau event each represent isotopic excursions following a minor mass extinction ] and associated rapid sea-level change, in addition to the larger extinction at the end of the Silurian . Each one leaves a similar signature in the geological record, both geochemically and biologically; pelagic (free-swimming) organisms were particularly hard hit, as were brachiopods, corals and trilobites, and extinctions rarely occur in a rapid series of fast bursts.

Isotopic Records

From the δ13Ccarb curve There are 3 major positive excursions in the Wenlock-Ludlow interval. They occur at the Sheinwoodian, Homerian and Ludfordian. Most of the Silurian isotope excursions coincide with distinct lithological and biotic changes. In low latitudes, intervals of high carbon isotope values are seen. Many of these cases can be characterized by the growth of reefs and the formation of extended carbonate platforms. These excursions can often be associated with a significant drop in sea-level. The information on δ13Corg is small though it appears that the major excursions shown in the δ13Ccarb curve are also shown in the δ13Corg curve. The amplitude of these varies. The excursions are attributed to enhanced burial of organic matter or to the weathering of exposed carbonate platforms. Though Cramer and Saltzman (2007) considered that the differences in the amplitudes between δ13Ccarb and δ13Corg in the early Sheinwoodian were from changing pCO2values in the atmosphere. The δ18O data in the Silurian is actually in higher resolution then Ordovician. This is based mostly on brachiopods. [7] The curve also shows the positive excursions that the δ13C curve also shows during both Silurian excursions and even the Hirnantian excursion. (Munnecke et al., 2003)

Current problems

Potential areas of future studies follow several problems in interpreting the data from this time. Some models for sea-level change during the Silurian remain controversial as are some of the Silurian segments of published Phanerozoic bathymetric curves. New 87Sr/86Sr isotope data from whole rock samples are presented by Gouldey et al. (2010-this volume)from the Ikla core in Estonia. The authors argue that the observed increase in the 87Sr/86Sr ratio is the result of weathering of radiogenic source rocks that were exhumed during the Early Silurian. The beginning of a major increase in this ratio during the Telychian coincides with a negative excursion in both the δ13Ccarb and δ13Corg values. There have been different hypothesis proposed in attempt to explain carbon isotope excursions during the Silurian. Even with these hypotheses there is no agreement on the steering mechanisms of these carbon isotope excursions. Some of these include glacial events, changes in oceanic circulation, or latitudinal changes in the formation of deep water and associated changes in deep-ocean circulation Positive δ18O excursions currently have several conclusions that may or may not be correct. Most say they are from intervals of lower sea-surface temperature and expanded ice sheets over Gondwana. While others state that it is due to intervals of high sea level with increased carbonate production and burial in epeiric seas. The connection between sea level and δ18O values isn’t well understood.

References

Resources

1)

2)

3)

4)

5)

6)

7)

8)

9)

10)

11)

12)

Notes

((reflist))