The Phytolith¹⁴C Puzzle: A Tale of Background Determinations and Accuracy Tests

Abstract

ABSTRACT. Over the past decades, analysis of occluded carbon in phytoliths (opaline silica mineral bodies that form in and between plant cells) has become a workhorse of paleoclimate and archaeological studies. Since different plant types exhibit distinctive phytolith morphologies, their assemblages are used in identifying vegetation histories or food culture adaptations. A few direct radiocarbon AMS measurements of phytoliths have been carried out, but these measurements are difficult due to the low concentrations of phytoliths in some plant species, and the small amount of C per phytolith (<2%). In addition, no phytoliths samples of a known 14 C age are available to verify measurement accuracy and precision, and to check sample preparation protocols. Background corrections are also difficult to address due to the lack of suitable material. In this work, we designed a procedure to quantify a suitable blank using SiO 2 powder samples (close to the opal structure, and free of 14 C). The full phytolith extraction showed high carbon contamination components: a) ~3 g of modern C and ~2 g of dead C. We also performed accuracy tests on large phytolith-occluded carbon samples extracted from soils and harvested plants. The unexpected 14 C ages in some of the results triggered further investigations of possible sources of carbon contamination.

Phytolith radiocarbon dating in archaeological and paleoecological research: a case study of phytoliths from modern Neotropical plants and a review of the previous dating evidence

“…The anomalously old 14 C ages obtained the past few years on phytoliths from recently collected and senesced grass leaves from the mid-western USA, France, and Australia, ranging from about 1800 to 8000 14 C yr. BP (Santos et al, 2010;Sullivan and Parr, 2008), were somewhat to far older than expected. However, at least some of those dates can be explained by phytolith persistence in soils for considerably longer periods than most other organic-derived matter, as even uppermost soil horizons may contain an unspecified number of phytoliths incorporated over hundreds of years and more (e.g., Fredlund and Tieszen, 1994;Bremond et al, 2004;Piperno, 2006:110;McMichael et al, 2012;Piperno et al, in press).…”

“…14 C dates on charcoal fragments ranging from 1250 BP to more than 2000 BP from the uppermost 12e20 cm of terrestrial soil in Brazil, Peru, and Ecuador (McMichael et al, 2012;Piperno et al, in press) further support the point that plant remains of that age that can survive for long periods are found in uppermost soil horizons. Furthermore, the older age range of 1740e2760 BP obtained from the Santos et al (2010) African soils was determined from the same phytoliths that had originally provided the younger range of 415e430 BP, after those samples were treated further for organic removal by oven baking at 500 C and another wet oxidization step. Much lower carbon content than before resulted along with melted and possibly carbon-depleted phytoliths (Santos et al, 2010).…”

“…During the past five years, 14 C determinations on phytoliths removed from recently harvested grass samples from North America, France, and Australia have returned inexplicably old ages ranging from about 2000 BP to 8000 BP (Sullivan and Parr, 2008;Santos et al, 2010). Recently harvested Chinese rice and millets also returned phytolith ages from a few hundred to 1000 years too old, depending on combustion temperature (Yin et al, 2014).…”

“…Recently harvested Chinese rice and millets also returned phytolith ages from a few hundred to 1000 years too old, depending on combustion temperature (Yin et al, 2014). These findings have led to considerable discussion of whether 14 C phytolith study provides accurate dates and if not, why not Parr, 2008, 2012;Santos et al, 2010Santos et al, , 2012aHodson, 2012;Yin et al, 2014). The discourse involves critiques and reviews of phytolith preparation techniques and procedures at radiocarbon facilities currently in practice for phytolith 14 C study, evaluating the likelihood they will remove extraneous carbon that may bias phytolith dates (Boaretto, 2009;Santos et al, 2010Santos et al, , 2012aCorbineau et al, 2013;Sullivan and Parr, 2012;Yin et al, 2014).…”

“…The discourse involves critiques and reviews of phytolith preparation techniques and procedures at radiocarbon facilities currently in practice for phytolith 14 C study, evaluating the likelihood they will remove extraneous carbon that may bias phytolith dates (Boaretto, 2009;Santos et al, 2010Santos et al, , 2012aCorbineau et al, 2013;Sullivan and Parr, 2012;Yin et al, 2014). Some investigators question the original source of phytolith carbon itself, suggesting that rather than being predominantly derived through photosynthesis from atmospheric CO 2 during plant growth, and thus being in equilibrium with the atmospheric 14 C content upon plant death as assumed in 14 C analysis, older soil organic matter (SOM) has entered plants via the root system and become incorporated onto or within phytoliths, skewing their ages (see discussions in Hodson, 2012;Santos et al, 2010Santos et al, , 2012aSullivan and Parr, 2012). As mentioned, in the Neotropics 14 C dates on phytoliths from various ancient sedimentary contexts have not appeared to have a too-old bias (see review in Piperno, 2006:125e131).…”