Searching for Sterile Soil…

Hi all!

Things are moving along, we have opened up a couple more units at the northern and southern extremes of the site (about 150m away from one other) which makes communicating, sharing equipment, getting  new paper work, etc a bit of a hassle. Texting back and forth has helped, as we are now out of yelling distance (thanks to a combo of distance, thickly planted coffee plants and some intense wind).

Anyway, one of the big issues to address this field season is to figure out exactly where some of the deeper cultural deposits are located, and to figure out patterns in the site stratigraphy that would help us figure out what is (capital S!) Sterile soil, meaning the strata under which no cultural material dares dwell. (or something like that).

While it might seem a little strange to be so concerned with finding a strata with NO cultural material in it, finding sterile soil is additionally important in this case because we know that the site contains precolonial remains below the colonial period deposits found closer to the surface (something we determined during preliminary excavations in 2010). Our deepest cultural deposit thus far is at about 150cm, and appears to date to the Late Classic period (AD 800-1000). In one test unit excavated 2010, we found Late Classic material at 150cm, directly under a potential Postclassic (AD 1000-1500) deposit, which was itself below two distinct colonial period occupation strata. However, many of the test units excavated thus far have yielded ONLY colonial period deposits, sometimes down to 130cm. Are we missing something? Do we need to dig deeper to make sure we don’t miss some deeply buried precolonial deposits?

We seem to have hit on something this last week that might help us figure all this out.

We noticed that all the test units opened so far have hit a strata of reddish coarse volcanic tephras (basically volcanic sand and gravel) at between 70-130cm below surface. There is never any cultural material in this strat or below it, although we do find some clayey/silty, dark strats sandwiched between the volcanic gravel strats that seem like they might be occupation layers (appear to actually just be episodes of moderate soil development between volcanic episodes prior to the occupation of the site; see image below of test unit profile for example).

Luckily, we have managed to get some carbon samples from the top, middle and bottom of this strata in a couple of test units this year. If we can carbon date this strat (which is found everywhere at the site, as it appears to have been deposited by a widespread volcanic event) and tie it to an eruption of the Volcan de Fuego, it would go a long ways towards defining the site chronology with absolute dates (complimenting, as well as potentially challenging some of our ceramic-chronology-derived strata dates).

Having defined our sterile soil (for the moment) is helping us get a handle on the natural and cultural deposits we have encountered at SPA. However, just in case, after we reach this sterile volcanic gravel strat we auger down an additional 150cm in two locations in each unit to make sure we do not miss a buried cultural layer, meaning we get down to between 250-275cm in each test unit. So far, no luck finding anything in the augers, so it seems we may have just found our sterile!

More info on all this as soon!