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Climate Change

Reconstructing climate history from sediments in the Gulf of Taranto, Italy

Article:

Goudea, M.S., Grauel, A., Tessaro, C., Leider, A., Chen, L., Bernasconi, S.M., Versteegh, G.J.M., Zonneveld, K.A.F., Boer, W., Alonso-Hernandez, C.M., De Lange, G.J., 2014.  The Glacial-Interglacial transition and Holocene environmental changes in sediments from the Gulf of Taranto, central Mediterranean.  Marine Geology 348, 88-102.  http://dx.doi.org/10.1016/j.margeo.2013.12.003

 

The Holocene epoch is specifically characterized by the change from a glacial period (approximately 11,700 years ago) to an interglacial period, like the period we currently live in.  The Holocene has had its share of climatic variability, with changes occurring on timescales of decades to thousands of years, including climatic events such as the Medieval Warm Period (~2000 – 1000 years before present (yr B.P.), estimated dates vary by literature) and the Little Ice Age (650 – 150 yr B.P.). The prevalence of these climate events has varied by location and the mechanisms causing them are not fully understood.  To best understand these variations, it is necessary to have high-resolution records in regions that are sensitive to climate change.  The Mediterranean region happens to be a region sensitive to both high and low latitude climate changes.  High latitude climate variability has been linked to the North Atlantic Oscillation (NAO), which is the configuration of high pressure and low pressure between the Azores and Iceland, respectively.  Low latitude climate variability has ties with the El Niño Southern Oscillation and Asian Monsoon.

The study area detailed in this article is the Gulf of Taranto, essentially a bay existing between the “toe” and the “heel” of the Italian peninsula (Figure 1).  Continuing the boot analogy, Italy “stands” on top of the Ionian Sea, and the Adriatic Sea is the body of water “behind” the boot.  The general circulation of water is such that the Western Adriatic Current carries water along the western boundary of the Adriatic Sea, around the “heel” of the Italian peninsula, and into the Gulf of Taranto, following the foot towards the “toe”.

Figure 1.  Bathymetric map of the study area, showing the Gulf of Taranto, the location of sediment cores and the West Adriatic Current.

Figure 1. Bathymetric map of the study area, showing the Gulf of Taranto, the location of sediment cores and the West Adriatic Current.

 

In this particular study, scientists reconstructed the climate history of the Mediterranean region using sediment cores collected from the continental shelf of the eastern Gulf of Taranto.  Sediment is deposited progressively over great lengths of time and often comes from land-based sources, being carried for long distances by rivers before being dropped and deposited on the seafloor.  Sediment can also be composed of the shells of tiny dead critters such as plankton, which ran down through the water column and accumulate on the seafloor.  In order to collect a sediment core minimal disturbance is necessary, therefore a tube is driven into the seafloor, preserving the layers of sediment (Figure 2).  The bottom of the sediment core contains the oldest sediment, whereas the top contains the youngest.  To reconstruct the climate history of this region, scientists analyzed the composition of the sediment, determined the age of the sediment, correlated the sediment cores, and determined where the sediment came from.

Figure 2.  An example of a sediment core showing younger sediments towards the top of the core and the oldest at the bottom of the core.

Figure 2. An example of a sediment core showing younger sediments towards the top of the core and the oldest at the bottom of the core.

 

After recovering sediment cores from the seafloor, a good first step is to be able to quantify what the sediment core is composed of.  This was done using Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and X-Ray Fluorescence (XRF).  Although these terms might sound like something straight out of the mouth of Sheldon Cooper on the television series “The Big Bang Theory”, they basically tell the scientists the elements (iron, manganese, calcium etc.) that compose the sediment.  Often the elements are reported as ratios.  For this study, composition was reported relative to titanium.  For example, the concentration of calcium (Ca) was reported as a ratio of Ca/Ti, Ti being titanium.

Age models are important for reconstructing climate history in a particular region.  In this study, one core was dated using radioactive isotopes 210Pb and 14C.  Essentially, a radioisotope is an unstable element.  Carbon can exist with 6 protons and 6 neutrons, or much less commonly with 6 protons and 8 neutrons.  The 2 extra neutrons cause carbon to be unstable.  14C becomes stable by emitting radiation and “decaying” to a stable “daughter”, 14N.  By knowing the rate at which carbon decays, scientists can measure concentration of 14C in a sample and compare the concentration to an estimated atmospheric 14C value.  By dating tiny shells containing carbon at multiple points from a sediment core, an age model can be constructed.  For this regional study, the scientists needed to be able to correlate sediment cores.  A comparison of measured Ca/Ti ratios through the length of the cores allowed for nice correlation.  The correlation can be observed in Figure 3, where high and low values of Ca/Ti can be observed and linked between each core.  With these links established, radioactive isotope dates from one core can be used to create age models for the remaining cores.

Figure 3.  Sediment cores showing Ca/Ti ratios with good correlation.  Sediment core DP30 was dated using radioactive isotope dating.  Age models were calculated for all sediment cores using the Ca/Ti correlation.

Figure 3. Sediment cores showing Ca/Ti ratios with good correlation. Sediment core DP30 was dated using radioactive isotope dating. Age models were calculated for all sediment cores using the Ca/Ti correlation.

 

Besides showing great correlation between cores, Ca/Ti ratios actually represent specific climatic conditions.  For instance, high Ca/Ti ratios in sediment represent high amounts of sediment composed of the shells of small organisms, where as low Ca/Ti ratios represent high sediment input from rivers.  High sediment input from rivers may suggest that rivers were flowing at greater rates due to increased precipitation, whereas high sediments composed of shells may indicate conditions favoring a plankton bloom.  Based on the measured concentrations of element age models, the climate history was reconstructed for the study area.  These results and conclusions can be divided into several time intervals, spanning from ~16,000 yr ago to today.

Glacial to Interglacial: 16,000 – 10,800 yr B.P.  

The climate history for this time period was mostly dry and stable, highlighted by 2 periods of increased input of land derived sediment, increased river flow and consequentially increased precipitation in Southern Italy.  The first was during the Bølling-Allerød Warm period (~14,700 – 12,700 yr B.P.) and the second during a short warm period during the Younger Dryas cool period (~12,800 – 11,500 yr B.P.).

 

Sapropel S1 – Saharan Warm Period: 10,800 – 7,000 yr B.P.

Due to increased primary production, the sediment cores indicated a higher concentration of black organic sediment.  It is interpreted that sea level was rising as a result of glacial melt and warming global climate.  Rising sea level in the Adriatic caused flooding of the continental margin to the North, possibly rerouting river drainage.  It is expected that the sediment of this period is from a more northern source due to this rerouting.

 

Late Holocene: 7,000 – 0 yr B.P.

The sediment cores record a constant increase in land-derived sediment.  This increase in land-derived sediment, and the rate of sediment input, is likely due to deforestation and the advent of agriculture by humans around 5,500 yr B.P.  Increasing climate wetness and the establishment of modern day atmospheric circulation is linked to the increase in sedimentation as well.  Dry periods in the Mediterranean during the Late Holocene have been correlated with positive phases of North Atlantic Oscillation, which determines the prevalence and intensity of the westerly winds.

 

The conclusions drawn in this study show how scientists can use sediment cores to reconstruct climate history at a regional scale.  Although the sediment cores were mostly from the eastern Gulf of Taranto, a small geographic region, the conclusions can be scaled to a larger geographic region.  The highlighted article discusses natural climate variability with timescales ranging from thousands of years to decades.  Although this article does not directly address human induced climate change, it provides great information on how scientists can reconstruct climate in the past using data from sediment cores.  Doing so allows scientists to understand how the Earth’s climate system operates and what falls into the natural range of variability, which in turn will help us better understand our current and future climates.

One last parting comment:  Although this article discusses natural climate variability, it does not negate the reality that we live in a time where humans are causing rapid changes in the Earth’s climate.  This reality will greatly stress our society on a time scale of decades.  Don’t believe me?  Have a look at the 2007 Intergovernmental Panel on Climate Change (IPCC) report, or better yet, keep up to date with the 2013 IPCC report, some of which has already been made available:  http://www.ipcc.ch/index.htm  After all, a good scientist should ask questions and objectively test their hypothesis!

 

 

Discussion

One Response to “Reconstructing climate history from sediments in the Gulf of Taranto, Italy”

  1. Great post!

    Posted by Judtih | January 16, 2014, 9:33 am

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