Here’s an interesting SST proxy from the Gulf of Mexico that I meant to report on last October, but is actually more timely now given our recent discussion of hurricanes. When I corresponded with Lloyd Keigwin around the time of the Juckes submission, he mentioned a “beautiful” unpublished high resolution core from Poore in the Gulf of Mexico, which, with a little detective work, turned out to be a core in Pigmy Basin which Poore and associates (Julie Richey) had put online as a poster.
Julie Richey made an excelllent presentation at my AGU session which I noted up here (comment=73623). While the proxy is interesting in itself as a high resolution proxy showing a MWP, it is of extra interest because it is located in the Gulf of Mexico just to the west of the Katrina-Rita hot spot as readily seen if you compare the location map below to the hurricane hot spots, for example, here. (The core in Richey et al PBBC1 is not shown in this location map but is also in Pigmy Basin nearby.)
Poore et al 2004 Figure 1. Location Map. Rita and Katrina “hot spots” were just to the east of the Pigmy Basin cores
At AGU, Richey presented a high resolution Mg/Ca proxy from core PBBC1, Pigmy Basin, Gulf of Mexico. The sedimentation rate was 43 cm/kyr, sampled at 0.5 cm intervals, achieving a resolution of about 12 years per sample. The radiocarbon reservoir used was 400 years. She reported that the MWP nearly a degree warmer than coretop; that the period 1000-1400 was as warm as the late 20th century while the LIA was about 2 degrees cooler than modern. She said that results were replicated in G ruber white and G ruber pink. These statements are also made in the online poster which also says that the SST record corresponds to sunspot maxima and minimia and that their are Strong links between the Icelandic Low and changes in GOM oceanography at 600 yrs BPRichey et al Figure 3 excerpted below (modern at left) shows a sharp decline in percentage G sacculifer (a warm-water proxy) around 1400AD, with a corresponding decrease in dO18. Richey et al say that
the relative abundance of G. sacculifer in Pigmy Basin Basin sediments indicates the influence of Caribbean waters in the GOM, and is related to the average position of the intertropical convergence zone (ITCZ), with increased abundances indicating a more northward position of the ITCZ (Poore et al. 2005).The àÅ½àⲱ8OG.ruber record is influenced by changes in both temperature and àÅ½àⲱ8O of seawater
She synchronized these changes to a synchronous change in sea-salt sodium (ssNa) in Greenland, related to development of a more pronounced (deeper) Icelandic Low, resulting in increased winter winds blowing from the North Atlantic onto Greenland, increasing the ssNa content of the ice, concluding that there was a close linkage between atmospheric circulation in the N. Atlantic and subtropical Atlantic and that there was a southward shift around 1400AD in the mean position of the ITCZ coinciding with the intensification of the IL.
Richey et al Figure 3. Relative abundance variations of the planktic foraminifer Globigerinoides sacculifer and àÅ½àⲱ8O measured G. ruber (white variety) in core PBBC1 plotted against calibrated years. Solid line is a 3-point running average. Bottom panel shows sea-salt-sodium (ssNa) record from GISP 2 ice core. The ssNa record is a 20-year smoothed version, resampled at 6-yr resolution, and is plotted against the time scale for GISP 2 (from Meeker and Mayewski, 2004).
Richey et al Figure 4 shows high values of Mg-Ca estimated SST, concluding that:
“⡔he high Mg/Ca values between 1000 and 1400 yrs BP indicate that SST in the northern GOM during portions of the MWP were warmer than near-modern SST.”⡍inimum Mg/Ca values between 300 and 200 yrs BP indicate that SST in the northern GOM during portions of the LIA were at least 2 to 2.5oC cooler than modern SST.
“⡔he Mg/Ca record does not show a significant feature at 600 yrs BP when major shifts are seen in mean values of the àÅ½àⲱ8O and G. sacculifer records. Thus the intensification of the IL and southward shift of the ITCZ at 600 yrs BP did not have a significant effect on SST in the northern GOM.
“⡉n general the Pigmy Basin record does not show a systematic relationship between temperature and salinity on multi-decadal to centennial timescales.
Richey et al Figure 4. Mg/Ca, àÅ½àⲱ8Ocalcite for G. ruber (white variety), and calculated àÅ½àⲱ8O seawater in PBBC 1. The Mg/Ca record is plotted with the corresponding SST values on the secondary axis, calculated using the equation Mg/Ca=0.449exp(0.09T) (Anand et al. 2003). Using both the àÅ½àⲱ8Ocalcite and derived SST values, we calculated àÅ½àⲱ8Oseawater using the equation SST(oC)=14.9-4.8*(àÅ½àⲱ8Ocalcite-àÅ½àⲱ8Osw)+.27 (Bemis et al. 1998).
Richey et al should be advancing to publication in 2007. She mentioned that one of the referees was extremely severe and ended up delaying publicaiton of results that usually proceed without interruption. For reference, here is Keigwin’s figure from the Sargasso Sea (note the different scale) also showing a pronounced MWP.
I presume that these elevated SST would be sufficient to generate lots of medieval hurricanes, in fact, lots of cat 4-5 medieval hurricanes. There has been some work done on “paleo-tempestology” attempting to quantify past hurricane activity from sand bars and similar evidence, which I may get to on some future occasion. The implication of a relationship between SST and hurricanes (which I find intuitively plausible notwithstanding concerns over HWC statistical methods) is that there should have been high levels of medieval Atlantic hurricanes.
Julie N Richey, Richard Z Poore, Benjamin P Flower, Terrence M Quinn, A 1400-year multi-proxy record of climate variability from the Northern Gulf of Mexico URL
Poore RZ, Quinn TM, Verado S. Century-scale variability of the Atlantic Intertropical Convergence Zone linked to solar variability. Geophysical Research Letters, 31, 1-4, 2004. URL SI – radiocarbon dates: