Climate Audit

David Black and associates have just (Oct 2007) published a new paper showing a reconstruction of SST from Cariaco sediments using Mg-Ca for the period 1221-1990 (thanks to Eduardo Zorita for alerting me to the study.) In this case, I am happily able to report that the relevant data was archived at WDCP contemporary with publication. Black had already archived data from two other important studies (Black et al 1999, Black et al 2004) in a timely fashion. It’s interesting to contrast Black’s exemplary archiving with the abysmal archiving for another new paper on bristlecones by Hughes and Salzer, noted up recently by Rabett (a paper which I’ll try to comment on some time), which uses information some of which has been unarchived for over 25 years – and where no information collected in the past 17 years has been archived.

Black et al 2007 has particular interest for readers here for several reasons. The Cariaco sediments have a high accumulation for ocean sediments (67 cm/kyr for core PL07-73 BC) and have an annual varve. This has a couple of important benefits: (1) dating precision can be achieved; (2) bioturbation is dramatically reduced (accumulation here being nearly an order of magnitude higher than Arabian Sea RC2730 for example). Secondly, Black has now published and archived near-annual results for four different Cariaco proxies: 1) Mg-Ca in Black et al 2007; 2) G Ruber dO18 and G Bulloides dO18 in Black et al 2004; and 3) weight G Bulloides in Black et al 1999. This latter proxy is not exactly the same as percentage G Bulloides used in the Arabian Sea studies of Anderson, Overpeck and Gupta, but it would presumably be pretty close.

It will also be interesting to compare these results to another high-resolution Mg-Ca seriespublished this year by Richey et al 2007, together with dO18, a study commended by Lloyd Keigwin. Richey presented this study at our AGU session – Mann didn’t like it very much. Tamino has complained about this study as well.

Before presenting Black’s discussion, here is my replot of his Mg-Ca data, also showing the earlier dO18 and G Bulloides data – something that arguably should have been done in Black’s paper (but at least he’s provided materials for others to do so quickly.) The first impression is obviously that the series don’t have much of a common signal over the past 1000 years. Given that Mg-Ca and dO18 give pretty interesting and consistent results on an Ice Age scale over the past million years, it’s a little disappointing to say the least that more of a common signal doesn’t emerge here. It’s not just a high-frequency failure: there’s little commonality in the low frequency either. This is also a disquieting result for results where authors purport to obtain “low frequency” correlations between data which is not nearly as well resolved in time as this data.

One other caveat: the Mg-Ca data (top panel) comes from a different (nearby) box core than the dO18 series. Is it possible that the dating is not all that consistent between the two cores? I don’t know.

Figure 1. Cariaco sediments. Top – Mg/Ca reconstruction of SST; middle – G Bulloides and G Ruber dO18; bottom – weight G Bulloides.

The correlations between the series are shown in the table below and are disappointing. Black states of tsuch comparisons: them):

A comparison of the Mg/Ca SST record to G. bulloides abundance, a proxy for upwelling and trade wind variability in the Cariaco Basin [Peterson et al., 1991; Black et al., 1999], reveals very little similarity between the two records. In particular, the large SST changes during the end of the MWP, the beginning of the LIA, and the mid to late twentieth century are not associated with corresponding changes in G. bulloides abundance in either PL07-71BC or PL07-73BC. However, Black et al. [1999] noted a nearzero correlation (r = 0.03) between G. bulloides abundance from a different Cariaco Basin core and local SSTs. A comparison of the G. bulloides abundance record from the core used for this study (PL07-73 BC) to instrumental SSTs over the period of instrumental overlap results in a weak, although statistically significant, correlation (r = 0.18, p <0.1).

Table 1. Correlations between different Cariaco High-Resolution Proxies

From the information in the top panel, Black reported:

On average, twentieth-century temperatures are not the warmest in the entire record, but they do show the largest increase in magnitude and fastest rate of SST change over the last 800 a.

This genuflection towards global warming is certainly not the first thing that an unbiased data analyst would take away from this information and in a way, reminds me of the now quaint genuflections towards Chairman Mao in Zhu (1973). Black observes that this record is relatively exceptional in its detail:

Varved, high deposition rate sediments deposited under anoxic conditions and an abundance of well-preserved microfossils result in one of the few marine records capable of preserving evidence of interannual- to decadal-scale climate variability in the tropical Atlantic.

and undertakes a calibration of the record against instrumental SST – something that hadn’t been to date in the ocean sediment community (due to low resolution).

No one to date has directly compared a downcore Mg/Ca record to historical instrumental data because there are very few areas with sufficiently high sedimentation rates where one can recover nonbioturbated sediments and high fossil foraminifera abundances. Long-term sedimentation rates in the Cariaco Basin are as much 1 m/ka, and sediments have been deposited under anoxic, nonbioturbated conditions for the last 12.6 ka, thus allowing one to compare and calibrate a suite of paleoceanographic proxies against historical instrumental data. …the Mg/Ca data were then compared to March–April–May average SSTs over the period of instrumental overlap (Figure 3a).

He then compares the record to other recent publications (but not to his own earlier results discussed in the graphic above or to the Richey et al 2007 Gulf of Mexico Mg-Ca result disliked by Tamino. Comparanda were:

• a high-resolution sclerosponge Sr/Ca-derived SST study from Jamaica (20 m below sea level) [Haase-Schramm et al., 2005]
• a planktic foraminiferal Mg/Ca SST records from just south of the Dry Tortugas in the northern Caribbean [Lund and Curry, 2006]
• Nyberg et al., 2002, noting a temporal offset between the two records that cannot be explained by age model differences alone near Bermuda. (Nyberg et al is one of the studies in Moberg)
• a shorter but high resolution coral Sr/Ca record [Goodkin et al., 2005]
• percent titanium data from Cariaco Basin sediments [Haug et al., 2001], said to show a similar pattern to Mg/Ca SSTs during the LIA but are distinctly different during the MWP. A
• a terrestrial lake record of gastropod d18O from the Yucatan Peninsula [Hodell et al., 2000] said to show a nearly identical LIA pattern, including a large drop in values at the beginning of the Maunder Minimum and a subsequent increase, but not the same trends during the MWP or the twentieth century

Black also did not take the opportunity to comment on the percentage Arabian Sea G Bulloides series used by Moberg. Below is a plot showing the difference between Black’s series at Cariaco – using well defined varved sediments – and the series by the Overpeck associates in the Arabian Sea. The Arabian Sea version is used in multiproxy reconstructions (Moberg, Juckes), but the Cariaco version is not. Can anyone give me a justification for using one and not the other that would pass even a first-order smell test?

Two G Bulloides Versions. Top – Cariaco; bottom – Arabian Sea.

References:

• David E. Black, Larry C. Peterson, Jonathan T. Overpeck, Alexey Kaplan, Michael N. Evans, Michaele Kashgarian, 1999. Eight Centuries of North Atlantic Ocean Atmosphere Variability, SCIENCE VOL 286 26 NOVEMBER 1999 Data
• David E. Black, Robert C. Thunell, Alexey Kaplan, Larry C. Peterson, and Eric J. Tappa. 2004. A 2000-year record of Caribbean and tropical North Atlantic hydrographic variability, PALEOCEANOGRAPHY, VOL. 19, PA2022, doi:10.1029/2003PA000982, 2004 Data
• David E. Black, Matthew A. Abahazi, Robert C. Thunell, Alexey Kaplan, Eric J. Tappa, and Larry C. Peterson, 2007. An 8-century tropical Atlantic SST record from the Cariaco Basin: Baseline variability, twentieth-century warming, and Atlantic hurricane frequency, PALEOCEANOGRAPHY, VOL. 22, PA4204, doi:10.1029/2007PA001427, 2007 Data