An age model is born...

Finally with the calibrated age we created an age model for one of the cores (HOLCAR03).

The deepest sample we dated was more than 40 ka (thousand) years old. As the core is almost 2 m long, we will have to use a different method to date the lowest part of it. Probably we'll use the oxygen isotopic composition of multiple samples of planktic foraminifera to create a curve that can be correlated with other paleorecords that have been dated using the U/Th series in coral samples. This way we'll be able to assigned dates to those sediments older than 60 ka.

Calibration

For the AMS 14C analyses, our samples were sent to the NationalIsotope Centre of the Institute of Geological and Nuclear Sciences Ltd (GNSScience) in New Zealand.

The resulting dates correspond uncalibrated years Before Present (BP). An uncalibrated BP date cannot be used directly as a calendar date, because the level of atmospheric 14C has not been constant during the past 60000 years, as the level of 14C is affected by variations in the cosmic ray intensity due to variations in the Earth's magnetosphere.

To calibrate our dates we used the software CalibRev 6.0 (Stuiverand Reimer, 1993) choosing the MARINE09 calibration data set. We also have to determine a reservoir age, this means to establish how old was the carbon of the water before it was integrated in the carbonate that formed the shells of the foraminifera. We choose a 400 years reservoir age and we keep it constant along the entire core.

How old are our samples?

After splitting the cores, collecting the sediment samples, and label, weight, and dry them, the moment has come to investigate their age. To do this we applied a Radiocarbon Dating method. This radiometric dating method uses the naturally occurring radioisotope 14C to estimate the age in those materials that contain carbon (e.g. organic matter, textiles, carbonates, wood). This method, however, can only be applied to date materials that are less than ~55-60 thousand years old, after this period the radioactive decay (the rate in which 14C decays into a non radiometric carbon isotope) is such that there is not enough 14C in the materials to calculate an age.

Radiocarbon measurements are traditionally performed by counting the radioactive decay of individual carbon atoms via gas proportional counting or via liquid scintillation counting. However, for small samples decay counting is relatively insensitive and subject to large statistical uncertainties.

In our case, as we dated samples of biogenic carbonate from the planktic foraminifera collected from the sediments, and as foraminifera are very small and their shells don’t weight much (in addition that only ~12% of calcium carbonate corresponds to carbon atoms) we can’t use proportional counting or liquid scintillation. Thus, we applied a technique called Accelerator Mass Spectrometry (AMS). This technique detects and counts the 14C atoms directly.  How? Samples for radiocarbon AMS measurements are incinerated. The resulting C02 is collected and reduce to a solid carbon (graphite) target for sputtering atomic carbon ions into a mass spectrometer, which allows dating samples containing only a few milligrams of carbon.

Cruise participant: Antoon Kuijpers

Antoon Kuijpers is a marine geologist specialised in marine sediment transport processes and late Quaternary (paleo)oceanography/climatology. Presently, his research focus is the Labrador Sea and adjacent Greenland waters, the Baltic Sea and the Caribbean. Previously, he also worked in the Barents Sea, North Sea, Northeastern and (sub)tropical Atlantic, and Indian Ocean.

Freeze drying samples

For some analyses (e.g. micropaleontological, compositional, etc.) it’s essential to freeze dry the sediment rather than oven-dry it.

Freeze-drying it’s preferable to oven drying because it removes the water content without greatly altering the physical structure of the sediment. As a result, sediments containing clay are much easier to handle (they remain friable) and diatom or foraminifera breakage, resulting from contraction of the sediment during drying, is greatly reduced. Furthermore, the formation of carbonate aggregates on the surface of calcareous microfossils due to carbonate re-precipitation, which may change their isotopic composition, is also prevented.

Cruise participant: Christof Pearce

I am a Ph.D. student at the Centre for Past Climate Studies of the Department of Geosciences at Aarhus University in Denmark. My research is on the reconstruction of past sea-surface conditions (temperature and sea-ice variability) in the North Atlantic Subpolar gyre region. For this I am using sedimentological, geochemical and micropalaeontological data with the main focus on diatom analysis.

Diatoms are unicellular algae and are generally the main constituents of the phytoplankton. As primary producers in the upper layers of the ocean, they play a vital role in the marine food web. The cell walls of diatoms have characteristic shapes and ornamentations depending on the species and are made up of silica, resulting usually in excellent preservation in the sediments. Because of the wide variety in species, all with their own ecological preferences (e.g. temperature, salinity, nutrient availability), diatom remains in sediments are ideal indicators of past sea-surface condition.  

At the moment I am studying a gravity core from offshore Newfoundland, that encompasses the Late Glacial to Early Holocene time period (from 12500 to 9500 years ago). In the core, the transition from glacial times to the current Holocene epoch is not smooth and is associated with numerous big climatic fluctuations such as the Younger Dryas, a cold interval when temperatures dropped almost back to glacial levels.

The diatom assemblages clearly reflect these changes as variations in sea-surface temperatures and sea ice presence.The results will be combined with analysis of benthic foraminifera and geochemical parameters to achieve a more comprehensive record. The study of these fluctuations is important to improve our understanding of past and present natural climate variability and will contribute to better future predictions.

Sampling the cores

After the cores arrived to the CORESTORE laboratory we split them longwise. Then we packed, labeled and deposited them in the cold room.

All the working halves* were taken to the GEOLOGICAL WELL SAMPLE LABORATORY. Here, with the help of Sebastian a Geology and Geography undergrad student working in the lab, we sliced each half section every 1 cm.

Dark gray layer: volcanic ashes?

The sediment composing the cores is high in carbonate content, hence the light color, however on the cores’ surface it was possible to distinguish some darker layers that we think are volcanic ashes. Some of these layers appear gray while others look greenish. The analysis of the elemental composition of the sediments will tell us why the difference in color and will confirm if those are, in fact,  pyroclastic particles.

Sebastian showing a slice of the core sediment

We use a CD to slice the cores. The physical and chemical characteristics of the polymer plastic use to produce CDs prevents the sticking of the sediment to   the CD surface, making the slicing process (that is already time consuming and a little tedious) much easier than when using aluminium or regular plastic cutters.

Each of the sample-slices was divided in 3, and each of these subsamples will be used for something different:

1. Elemental analysis (composition of the sediment) and magnetic susceptibility

2. Planktic and benthic foraminifera (assemblage changes, stable isotopes, etc)

3. Sediment characteristics (granulometric analysis)

At the end, the linning is empty and we have 265 x 3 little bags full of mud.

The end of the core, finally!!!

*The half of the core that is used for all the analyses is commonly referred as the Working half. The half that is keep intact and preserved in the cold room is the Archive.

A little more about our project

Cruise rute from Tuxpan (Mexico) to the Yucatan Strait

The cruise's focal objective was to obtain sediment samples—by gravity and box core retrieval—from depths between 500 and 2000m along the Yucatán Strait,which is an ocean channel extending for 217 km between Cabo Catoche (Mexico) and Cabo San Antonio (Cuba). This strait represents the boundary between the Caribbean Sea and the Gulf of Mexico.

Oceanographically speaking, this is a very interesting area, where water masses advecting from the Caribbean Current, derived from the North and South Equatorial Current, enter the Gulf of Mexico to form the Loop Current, outlining the foundation of the Gulf Stream. Thus, sediments from the Strait of Yucatan may contain a record of the past oceanographic variability that affected the strength and characteristics of the Gulf Stream in previous times.

With sediment from our cores, we are working towards generating a multi-proxy record of high resolution to reconstruct past climate and oceanographic variability in this region.

Finalcial support from this cruise was provided by the Dansk Center for Havforskning (Danish Center for Marine Research)  in their 2010 applicatuion for ship-time funding.

NEWS

After some months without any updates on the blog it's time to start posting again. We've started the analyses of the recovered cores and it's now time to tell you about it. Also,  we have made some changes to the blog:

1. To make it easier for everybody to understand what we did during the cruise, we've decided to translate all entries regarding the trip to English (you can still find the original version, in Danish, German or Spanish, as a comment to each particular entry) .

2. We are also starting a series of notes about each one of the participants. Once a week I'll post some info about one researcher or student, telling you about them, what they do and where are they now.

3. You will also find new pictures from before, during and after the cruise. We plan to change this pictures at least one a month.

The adventure is finished

A nice view of Tuxpan from the B.O. Justo Sierra

We arrived back in Tuxpan at midnight of March 29th.

Next morning all the equipment was load back in the container and we started to say goodbye to the crew, to our other fellow students and scientits and to the warm weather.

I hope the container will arrived in Denmark without delays or problems and we´ll have samples to work on for a long time.