Scale of time-averaging in archaeological shell middens from the Canary Islands

Abstract

Characterizing the degree of disturbance in archaeological deposits is critically important for archaeologists assessing foraging strategies, environmental conditions, or behavior patterns of ancient human groups. Qualitative techniques (e.g. micromorphology analysis) have previously been applied to assess the degree of disturbance (age-mixing) in archaeological sites; however, quantitative dating of material in the sites provides a more robust assessment of potential age-mixing. Unfortunately, because of budget constraints, archaeologists are frequently forced to rely on few quantitative age dates for an assemblage, thus obfuscating the signal of age-mixing of the deposit. The development of an affordable and rapid carbonate-target accelerator mass spectrometry (AMS) radiocarbon (14C) dating method provides a cost-effective way to retrieve more quantitative dates from carbonate material in archaeological assemblages to assess the degree of age-mixing in the deposit. This study tests this new technique and dates numerous harvested marine limpet shells from archaeological sites in the Canary Islands to determine whether there is multidecadal to multicentennial age-mixing. A total of 58 shells retrieved from six sites and three islands yielded uncalibrated radiocarbon ages ranging from 2265 ± 40 to 765 ± 35 BP, coinciding with the time of prehistoric human occupation in these islands. While most shells from the same stratum showed statistically equivalent ages, in some cases we detected age ranges that exceeded the imprecisions from analytical errors. This investigation is one of the first to quantitatively illustrate that shells retrieved from depth intervals without evident stratigraphic disturbance do not always contain contemporaneous remains and, therefore, dating each specimen is valuable for developing further paleoclimatic and paleoanthropological inferences. This study presents the first report of carbonate-target 14C ages from archaeological shell middens, and suggests that this novel radiocarbon methodology can be applied to these sites, thus allowing the generation of a more comprehensive chronology.

COMPARING DIRECT CARBONATE AND STANDARD GRAPHITE 14C DETERMINATIONS OF BIOGENIC CARBONATES

“…We compiled 60 additional paired determinations generated at UCI or the Australian Nuclear Science and Technology Organisation (ANSTO) from Bush et al (2011), New et al (2019, Parker et al (2019), andHua et al (2019). The respective publications provide the lab procedures and methods used for the additional paired determinations.…”

“…An increasing variety of scientific investigations require a large number of radiocarbon analyses to address their underlying research questions, as exemplified by recent studies assessing the degree of time-averaging in natural or anthropological shelly accumulations (Kowalewski et al 2018;New et al 2019;Parker et al 2019;Albano et al 2020). These types of studies are generally constrained by their analytical budget rather than by the number of samples suitable for analysis, whereas some are limited by the size of the targeted specimens.…”

“…In response to this growing need, a direct carbonate AMS 14 C sputter method was developed by Longworth et al (2013) that allows submilligram samples of carbonate powder to be analyzed quickly and efficiently. Several publications have highlighted the utility of direct carbonate 14 C dating where it has been used on its own or in combination with amino acid racemization to determine time-averaging in taphonomic studies (Dominguez et al 2016;Kosnik et al 2017;Ritter et al 2017;Parker et al 2019;Albano et al 2020) or coupled with standard precision 14 C and uranium/thorium dating to determine coral age distributions (Grothe et al 2016).…”

Chronology, time averaging, and oxygen isotope composition of harvested marine mollusk assemblages from Ifri Oudadane, northeast Morocco

“…Shells are often preserved in so-called middens, which can be defined as ancient human garbage dumps with high concentrations of discarded foods (e.g., shells) and other remains accumulated throughout time. However, the study of archaeological shell concentrations can be challenging, because an array of natural and anthropogenic processes are involved in the formation of middens, and these processes are difficult to disentangle (Parker et al, 2020).…”

“…To properly interpret the stratigraphy and geochronology of mollusk shell assemblages, it is essential to quantify the scale of time averaging, or the mixing of remains of different ages occurring in the same stratigraphic layer (Kidwell and Bosence, 1991; Kowalewski, 1996; New et al, 2019; Parker et al, 2020). Failing to recognize time averaging can lead to the misinterpretation of ancient assemblages that may have the appearance of being isochronous while they are actually diachronous (Kidwell and Bosence, 1991; Kowalewski, 1996; New et al, 2019; Parker et al, 2020). There are various natural sedimentary processes that can lead to the mixing of noncontemporaneous remains in the same horizon.…”