In our recent discussion of the Arabian Sea G Bulloides series, I noticed a remarkable increase in the coarse fraction percentage in the top 1.3 cm of the critical RC2730 core.
Willis has also commented on this. In core 2730, there is a correlation of 0.91 between the fraction of coarse particles and the percentage of G Bulloides.
This raises a couple of interesting questions:
- Is there some climatic reason for the increase in the percentage of coarse particles at the top of the core?
- Or alternatively, are there possible non-climatic reasons for the increase in coarse particle percentage?
In either event, does this have any implications for how one interprets percentage G Bulloides series? This leads, as so often, into many interesting byways and the answers are not entirely the ones that one would expect.
First, here is my plot showing the increase in percentage G Bulloides, percentage carbonate and percentage coarse fraction for the nearby cores RC2735, RC2730 and 723A. Willis has observed that RC2730 is deeper than RC2735 and that quite different currents may apply. 723A is deeper still. (See here for Willis’s graphs).
Willis observed the following:
As you can see, RC2735 is at an escarpment the edge of an upper plateau, which falls down to a lower plateau where RC2730 is located. Because of this, the coarse fraction will be very sensitive to the exact direction and depth of the currents in the area. If the current is running across the escarpment, there will be a large speed difference over the two areas, which will change the coarse fraction. Alternatively, the presence of the escarpment may mean that the deeper plateau is swept by an entirely different current than the upper plateau.
Next, the coarse fraction is related to the dissolution rate of the shells. There’s a discussion of this in the paper Coarse fraction fluctuations in pelagic carbonate sediments from the tropical Indian Ocean: A 1500-kyr record of carbonate dissolution. In that paper, they say the changes in coarse fraction:
… probably result from changes in surface productivity associated with monsoon variability. Dissolution at this site may be ultimately controlled by the oxidation of organic matter which appears to be incorporated into the sediments in greater quantity during periods of weak SW monsoon and/or increased dry NE monsoon
However, if monsoon strength were the only reason, we’d expect the two records to move in parallel since the winds will be quite similar at two oceanic sites that are only 10 km apart … but they don’t move in parallel.
Finally, the fact that the increase is all in the top couple of centimetres at least allows for the possibility that the fines have been partially washed out of the top of the sampled core.
Without necessarily disagreeing with any of these observations, I’ve also identified literature that discusses an interesting connection between bioturbation and sediment coarse fraction.
The top few cm of ocean sediments are “mixed layer” in which mixing processes are taking place that are not taking place in sediments that are buried by 30 cm of overlying material. Thus the sediment for 1970 AD obtained by an paleoceanographer 10000 years from now would not be the same as the present sediment – its makeup would be altered by mixing.
McCave (1988), entitled “Biological pumping upwards of the coarse fraction of deep-sea sediments” reports that the coarse fraction in the mixed layer is older than the fine fraction and the difference in age can be substantial. This process was summarized by Sachs et al 2000 as follows:
The impact of bioturbation on radiocarbon dates is discussed in a number of articles by Thomson. Thomson et al 1995 says:
Thomson et al 1995 showed the following interesting illustration of this effect, in which the mixed layer is dated anomalously old – which they attribute not any errors, but to the higher proportion of older coarse material in the mixed layer, which then makes the mixed layer seem “too” old.
The data and arguments of Wheatcroft (1992) show that entirely different bioturbation intensities apply to the large foraminifera as compared with the bulk finer sediments. The biological diffusivity of the fine (<16 μm) is >1 cm2/yr whereas that for sand-sized particles is < 0.1 cm2/yr. Large foraminifera (>150 μm) have also been shown to be less deeply worked down into a recent turbidite surface than natural radionuclides, in a case where both the foraminifera and radionuclides have been deposited since emplacement 1 ka ago (thompson et al 1988; Thomson and Weaver 1994).
Brown et al 2001 characterize the results of Thomson et al 1995 as follows:
Thomson et al. (1995) concluded that the remaining possibility was particle-size induced differential mixing by benthic organismsa mechanism also used to explain the presence of lag layers of coarse (>2 mm) particles derived from a glacial gravel sediment found in a mud deposit several centimeters above the original horizon of deposition (McCave 1988). In essence, this hypothesis proposes that mixing ceases to be completely random when burrowing organisms encounter large sediment grains, as less energy is required to move such grains upwards than to push them downwards, whilst fine sediment then falls into the void thus created. If this mechanism is applicable to the coarse grained foram fraction of deep-sea sediment, dif\fering rates of transport out of the SML for the fine and coarse fractions will result in an age offset
This could have important implications for the RC2730, RC2735 and 723A results. The highly correlated increase in percentage G Bulloides and percentage coarse fraction both occur in the top 1.2 cm of RC2730 – which would be well within the mixed layer of most sediments. Anderson et al 2002 note in passing for RC2730:
the uppermost four samples, including the replicated 0- to 2-mm sample, were too young (less than zero age), which can be explained by the presence of bomb radiocarbon that has mixed downward over the upper 10 mm
Gupta et al 2003, the successor article, say of 723A (which, at 808 m, is deeper than RC2730):
The bioturbation smoothing that would normally affect fine-scale variability is minimal at this site owing to the strong oxygen-minimum zone that exists in the Oman margin.
This assertion seems inconsistent with the prior statement in Anderson et al 2002 (but this may require amendment.)
You can see how the potential knock-on effect of the mixing problem – whether through bioturbation or mechanical means. The G Bulloides series has been extracted from the coarse fraction (plus 150 μm). Suppose there has been upward biological (or other) pumping of the coarse fraction relative to the fines. The offsets in radiocarbon dates in other places can be several hundred years.
We know that the coarse fraction increases dramatically in the top 1 cm of RC2750. What if this represents “biological pumping upward” rather than climatic change? And at this point, we have evidence of the biological pumping upward process but not of a climatic relation between global temperature and coarse fraction at RC2730. Thus at RC2730, where the accumulation rate is only 9 cm/kyr according to the dating in Anderson et al 2002, the coarse fraction in the 0-2 mm sample might be integrating coarse fraction over the past 400 years or so and be a LIA signal as opposed to a 1986 signal.
This is the sort of thing that one would expect specialists to sort out in attempts to replicate the use of percentage G Bulloides as a proxy for temperature. Unfortunately, to date, this hasn’t occurred. This study is a one-off. The author of the only related study – G Bulloides at Cariaco – condemned the use of G Bulloides at his site for large-scale temperature reconstructions (his G Bulloides indicated a warm MWP.)
David M. Anderson,1* Jonathan T. Overpeck,2 Anil K. Gupta3, 2002. Increase in the Asian
Southwest Monsoon During the Past Four Centuries. Science.
P. Ascough, G. Cook, and A. Dugmore, 2005. Methodological approaches to determining the marine radiocarbon reservoir effect, Progress in Physical Geography, December 1, 2005; 29(4): 532 – 547.Louise Brown, Gordon T Cook, Angus B MacKenzie, John Thomson, 2001. RADIOCARBON AGE PROFILES AND SIZE DEPENDENCY OF MIXING IN NORTHEAST ATLANTIC SEDIMENTS, RADIOCARBON, Vol 43, Nr 2B, 2001, p 929937 url
I. N. McCave, 1988. Biological pumping upwards of the coarse fraction of deep-sea sediments . Journal of Sedimentary Research; January 1988; v. 58; no. 1; p. 148-158
Sachs et al 2000, Alkenones as paleoceanographic proxies. G3 url
Thomson. Thomson et al 1995 Radiocarbon Age Offsets in Different-Sized Carbonate Components of Deep-Sea Sediments, Radiocarbon 1995.
Robert A. Wheatcroft, 1992. Experimental Tests for Particle Size-Dependent Bioturbation in the Deep Ocean, Limnology and Oceanography, Vol. 37, No. 1 (Jan., 1992), pp. 90-104
Gupta et al 2003. Nature.
S. Barker, W. Broecker, E. Clark; I. Hajdas; G. Bonani, NON-CONCORDANT 14C AGES OF CONTEMPORANEOUS PLANKTONIC FORAMINIFERA url