This blog summarises the recent JET publication titled “Orbital pacing and secular evolution of the Early Jurassic carbon cycle” by Storm et al., (2020)
This photograph was taken at the site of the original Llanbedr (Mochras Farm) borehole.
Researcher Dr. Marisa Storm and her JET Project colleagues recently published a study based on an analysis of core material from the Mochras borehole. The main objectives of the study were:
- To produce a “continuous and biostratigraphically well-defined δ13CTOC record from uppermost Rhaetian (Triassic) to Pliensbachian (Lower Jurassic) strata”.
- Examine the carbon isotope excursions within this record.
The Mochras borehole (Cardigan Bay, Wales) was drilled in the late 1960’s. The coring operations recovered ~1300 meters (that’s over 4 times the length of the Eiffel Tower!) of Upper Triassic to Lower Jurassic material.
This map of the UK shows the location of the Mochras borehole, West Wales. Hesselbo et al. (2013); Coward et al., (2003).
Mochras sediments were deposited in the Cardigan Bay Basin, which, in the Early Jurassic, was located within an ancient middle latitude seaway (think present day Californian latitudes). The location of the basin is reflected by a marine succession of interbedded calcareous mudstone, strongly bioturbated calcareous siltstone and silty limestone.
Storm et al. (2020) used core samples to measure isotope ratios of total organic carbon (δ13CTOC). Natural isotopes are variations of the same chemical element where the atomic nucleus has different numbers of neutrons. All carbon atoms have six protons in their nucleus, but some have six neutrons (C12) and others have seven neutrons (C13).
Unfamiliar with isotopes? If so, this video provides a brief explanation to carbon isotopes:
[youtube https://www.youtube.com/watch?v=wMx1l86XFLU]
The isotopic ratio of the organic carbon (δ13CTOC) within a rock sample reflects the proportion of the lighter (C12) isotopes to the heavier (C13) isotopes. This ratio is controlled by the source of the organic carbon, which is related to the climate and environment in which it originated.
Fluctuations in (δ13CTOC) between 0.5‰ and 2‰ have been linked to astronomically controlled climate changes. Storm et al. (2020) correlated perturbations in the Mochras carbon isotope record with long-eccentricity cycles, which are a type of Milankovitch cycle. Evidence of Milankovitch cycles in the sedimentary record reflect changes in Earth’s orbit that affects the amount of radiation reaching the surface. These variations in radiation affect the climate on a scale of tens to hundreds of thousands of years.
Never heard of Milankovitch cycles? If so, this short video provides a brief introduction:
[youtube https://www.youtube.com/watch?v=iA788usYNWA]
These cycles can be observed and measured in the Mochras record because they represent changes in lithology and geochemistry. Because the cycles are predictable and stable in duration, they can be used to very accurately date the sedimentary record, thus giving scientists an accurate timeline on which to describe any other environmental changes they observe/measure.
Eccentricity is controlled by the shape of the Earth’s orbit, which varies from circular to elliptical. Eccentricity modulates precession (a different Milankovitch cycle caused by the wobble of the Earth’s axis) and leads to cyclical changes that vary on 405 kyr and 100 kyr time scales. Thus, Dr. Storm and colleagues assigned 405 kyr to each long eccentricity cycle within the Mochras section, resulting in durations of 8.8 Myr and 6.6 Myr for the Pliensbachian and Sinemurian stages, respectively.
Time-scale for the Mesozoic Era showing the age model assigned by Dr. Storm and her colleagues for the Sinemurian (6.6 Myr) and Pliensbachian (8.8 Myr) stages. These are the second and third stages of the Jurassic.
During the high precipitation intervals associated with eccentricity maxima, there is enhanced weathering and therefore more run-off from the terrestrial into the marine environment. This increases the availability of nutrients in marine waters and causes algal blooms. Strong/prolonged algal blooms in the upper water column lead to stratification (i.e., no or little mixing) and result in bottom waters being depleted of oxygen (i.e. anoxic). Anoxic bottom water conditions better preserve the organic matter that is produced in the photic zone (upper water column where sunlight penetrates), exported to the sediment surface, and deposited.
In contrast, dry seasons (eccentricity minima) are characterised by increased carbonate production, reflected in the geological record by extensive limestone deposits. During dry seasons, there is increased oxidation and degradation of organic matter in terrestrial and marine environments. This reduces the deposition and preservation of sedimentary organic matter in the geological record.
The more stable climatic conditions associated with eccentricity minima resulted in more stable nutrient input into the marine realm. This is reflected in the geological record from Mochras by less extreme fluctuations in the organic carbon content of the sediments. These climatic changes are also preserved in the geological record by changes in the carbon isotope ratio. During eccentricity maxima, the extreme fluctuations in precipitation and changing bottom water/surface sediment conditions are reflected in perturbations in the carbon isotope record (called excursions). The negative shifts in δ13CTOC correlate with anoxic bottom waters during “monsoon-like” wet intervals.
The tuned δ13CTOC record from the Mochras borehole showing 405 kyr long eccentricity cycles in the Early Jurassic. The grey line represents the raw δ13CTOC data and the red line is the filtered output for the 405 kyr cycles (i.e. all the patterns except for 405 kyr cycles have been removed from the raw δ13CTOC data). The stage names and ammonite zones are labelled on the left. Storm et al., (2020).
Overall, reconstructing Milankovitch cycles in the marine sediment record is a very useful stratigraphic tool because all of the subsequent work, including studies on environmental and climate change during the Early Jurassic, hinges on this initial temporal framework. Therefore, the age model that Dr. Storm and her colleagues generated here using eccentricity cycles is an extremely important contribution to the JET project as a whole and to the wider scientific community.