Influence of increasing combustion temperature on the AMS 14C dating of modern crop phytoliths

Abstract

Several attempts have been made to directly date phytoliths, but most 14C results are not consistent with other independent chronologies. Due to the limited dataset, there is not a clear explanation for these discrepancies. Herein, we report the 14C ages of phytolith-occluded carbon (PhytOC) from contemporary rice and millet crops that were combusted at different temperatures to investigate the relationship between the combustion temperature and resulting 14C age. Our results show that the 14C age of PhytOC increases directly with combustion temperature (up to 1100°C) and results in age overestimations of hundreds of years. Considerably older ages are observed at higher temperatures, suggesting that it may be possible to distinguish between two fractions of organic carbon in phytoliths: labile and recalcitrant carbon. These findings challenge the assumption that PhytOC is homogeneous, an assumption made by those who have previously attempted to directly date phytoliths using 14C.

Dating rice remains through phytolith carbon-14 study reveals domestication at the beginning of the Holocene

“…14 C determinations presented here showed consistent results between the phytoliths and other dated materials from the same level or context, validating the reliability and accuracy of phytolith 14 C dating. We find no evidence to support arguments from a few investigators (38)(39)(40) that old carbon absorbed from soils bias phytolith dates (see also SI Text). Our results here, combined with others from archaeological and paleoecological contexts and modern plants, indicate that phytolith ages are not significantly biased by extraneous carbon and can be used to establish reasonable chronological sequences in archaeological research (16)(17)(18)22).…”

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

“…Most recently, Yin et al (2014) reported that modern phytoliths from rice and millets increased in 14 C age from a few hundred to 1000 years with increasing combustion temperatures (range: 600e1400 C), with the age offset becoming the greatest at the highest temperature. They also concluded that two sources of carbon were present; however, in contrast to Santos et al (2012a) both sources were hypothesized to be intrinsic to the plant, deriving from different places in cells.…”

“…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).…”

“…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).…”

“…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). 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).…”