C. Fred T. Andrus

Chapter 2 – Mid-Holocene climate and culture change in coastal Peru

Publisher Summary This chapter reviews the history of study and the current status of Mid-Holocene climatic and cultural change along the Peruvian coast, with a focus on major transitions at ca. 5800 and 3000 cal yr BP that correlate temporally with changes in El Nino Southern Oscillation (ENSO) frequency. It begins with presenting the Peruvian archaeological record of Holocene El Nino frequency variation, considering several hypotheses to explain the data. Based on the archaeological record, it is concluded that for some time prior to 5800 years ago, the coast of Peru, north of 10°S latitude, was characterized by permanent warm water. From these data, it is hypothesized that El Nino did not operate for some period before 5800 cal yr BP; after that time, conditions as essentially the same as today were seen. Present-day climatic variability on interannual time scales in the tropics is dominated by ENSO, which involves both the atmosphere and the ocean in the tropical Pacific (e.g., Maasch, in press). Through teleconnections, extratropical climatic variability on these time scales is also impacted by ENSO. Continuous natural Holocene paleoclimate archives from northern Peru, Ecuador, and the eastern tropical Pacific Ocean are difficult to find, privileging anthropogenic deposits from archaeological sites. Although precisely dating these records is difficult, climatic change determined from them is consistent. The regional paleoclimate records are presented.

Molluscan radiocarbon as a proxy for El Niño–related upwelling variation in Peru

Sequential measurements of molluscan radiocarbon are demonstrated to be an effective proxy of seasonal and El Niño–related upwelling variation in coastal Peru. A Trachycardium procerum valve from southern Peru was measured through ontogeny for radiocarbon via accelerator mass spectrometry (AMS) as well as δC and δO. A specimen collected in 1984 near Casma, Peru (~9.30°S) grew before and during the 1982–1983 El Niño/southern oscillation warm event. Shell morphology recorded El Niño warming as a shallow growth break with subsequent realignment of aragonite crystal microstructure. The presence of this growth pattern indicates that shell material was precipitated during the warm event and that each AMS sample could be independently identifi ed to represent a defi ned period in the El Niño/southern oscillation cycle. Samples taken from portions of the shell precipitated prior to the El Niño warm event (before the diagnostic growth break) had a mean value of 99.8 percent modern carbon (pMC), with a maximum seasonal range of 2.1 pMC. During warming, as indicated by a negative excursion in δO and the growth break, there was an abrupt increase to 107.9 pMC. Aragonite precipitated near the margin of the valve, after the El Niño/southern oscillation event concluded, had radiocarbon values approaching those present before the growth break. We attribute this radiocarbon distribution to variations in vertical mixing of surface and deeper upwelled water of greater ventilation age. As an El Niño event begins and the thermocline deepens, less deep water reaches the surface. Thus, radiocarbon values in shell precipitated during El Niño appear younger (more positive) relative to non–El Niño periods, which represent periods of more intense upwelling. The results from the modern specimen validate the use of molluscan radiocarbon as a proxy of upwelling conditions related to El Niño/southern oscillation and suggest the utility of similar analysis of more ancient valves in both oceanography and paleoclimatology.

El Niño impact on mollusk biomineralization-implications for trace element proxy reconstructions and the paleo-archeological record.

Marine macroinvertebrates are ideal sentinel organisms to monitor rapid environmental changes associated with climatic phenomena. These organisms build up protective exoskeletons incrementally by biologically-controlled mineralization, which is deeply rooted in long-term evolutionary processes. Recent studies relating potential rapid environmental fluctuations to climate change, such as ocean acidification, suggest modifications on carbonate biominerals of marine invertebrates. However, the influence of known, and recurrent, climatic events on these biological processes during active mineralization is still insufficiently understood. Analysis of Peruvian cockles from the 1982–83 large magnitude El Niño event shows significant alterations of the chemico-structure of carbonate biominerals. Here, we show that bivalves modify the main biomineralization mechanism during the event to continue shell secretion. As a result, magnesium content increases to stabilize amorphous calcium carbonate (ACC), inducing a rise in Mg/Ca unrelated to the associated increase in sea-surface temperature. Analysis of variations in Sr/Ca also suggests that this proxy should not be used in these bivalves to detect the temperature anomaly, while Ba/Ca peaks are recorded in shells in response to an increase in productivity, or dissolved barium in seawater, after the event. Presented data contribute to a better understanding of the effects of abrupt climate change on shell biomineralization, while also offering an alternative view of bivalve elemental proxy reconstructions. Furthermore, biomineralization changes in mollusk shells can be used as a novel potential proxy to provide a more nuanced historical record of El Niño and similar rapid environmental change events.

Using Oxygen Isotope Sclerochronology to Evaluate the Role of Small Islands Among the Guale (AD 1325 to 1700) of the Georgia Coast, USA

ABSTRACT In this article, we present the results of sequential oxygen isotope analysis performed on hard clam, Mercenaria spp. (n = 5) and eastern oyster, Crassostrea virginica (n = 13) valves excavated from Pumpkin Hammock (9MC350). These data are used to evaluate Guale models of settlement and subsistence along the Georgia coast during the late pre-Contact and early Colonial eras (ca. AD 1325 to 1700). Season of collection data indicate shells were collected and deposited on the island during all four seasons, suggesting year-round occupation of the site. In addition, oxygen isotope values imply the Guale exploited mollusks from habitats over a wide range of salinity. We interpret this to signify that the Guale ranged broadly over the coastal landscape and that they were able to exploit these distant resources due to their use of watercraft. Finally, we place these findings within the broader context of the changing social landscape during the late pre-Contact and Colonial eras of the Georgia coast.

Marine radiocarbon reservoir age variation in Donax obesulus shells from northern Peru: Late Holocene evidence for extended El Niño

For at least 6 m.y., El Nino events have posed the greatest environmental risk on the Peruvian coast. A better understanding of El Nino is essential for predicting future risk and growth in this tropical desert. To achieve this we analyzed archaeological and modern pre-bomb shells from the surf clam Donax for the radiocarbon reservoir effect (ΔR) to characterize late Holocene coastal upwelling conditions in northern Peru (8°14′S). Mean ΔR values from these shells suggest that modern upwelling conditions in this region were likely established between A.D. 539 and A.D. 1578. Our radiocarbon data suggest that upwelling conditions ca. A.D. 539 were less intense than those in modern times. The observed coastal water enrichment in 14 C may be consequence of frequent strong El Nino events or extended El Nino–like conditions. These ΔR-inferred marine conditions are in agreement with proposed extended El Nino activity in proxy and archaeological records of ca. A.D. 475–530. Extended El Nino conditions have been linked to political destabilization, societal transformation, and collapse of the Moche civilization in northern Peru. A return to such conditions would have significant impacts on the dense population of this region today and in the near future.

Modeling molluscan marine reservoir ages in a variable-upwelling environment

Abstract Changes in marine upwelling can affect the radiocarbon content of seawater and thus affect the marine radiocarbon reservoir age, R. These radiocarbon variations are preserved in mollusk shell carbonate. Shell-based estimates of R in a variable-upwelling environment can be biased by (1) changes in molluscan growth rates due to fluctuating environmental conditions and (2) time averaging during sampling due to homogenization of days or weeks of precipitated carbonate. We modeled the growth, radiocarbon content, and radiocarbon sampling of two Peruvian mollusks, Argopecten purpuratus (bay scallop) and Mesodesma donacium (surf clam), to quantify these potential biases. Argopecten purpuratus grows year round, but M. donacium prefers cold conditions and its growth rate decreases in summer. Radiocarbon assays by accelerator mass spectrometry on multiple ∼1 mg samples of a model A. purpuratus shell can capture the full range of annual R variation; similar sampling of a model M. donacium shell only captures ≤75% of this range. Given an annual R variation of 530 14C yr, the mean R calculated using a group of ∼1 mg carbonate samples from an A. purpuratus shell is within 30 14C yr of the actual mean R; that of a M. donacium shell may be skewed up to 140 14C yr older than the actual marine mean. If growth tolerances and parameters of mollusks used for R analyses are considered, it may be possible to correct for these biases and improve the accuracy of marine radiocarbon chronometry.

Exploring Oyster (Crassostrea virginica) Habitat Collection via Oxygen Isotope Geochemistry and its Implications for Ritual and Mound Construction at Crystal River and Roberts Island, Florida

ABSTRACT Research at Crystal River and Roberts Island Shell Mound Complex, on the western coast of Florida, USA, offers a quantitative assessment of the temporality of shell deposit construction, Native subsistence practices, and mobility patterns through stable oxygen isotope data from eastern oyster (C. virginica). The δ18Owater values of oysters vary synchronously with salinity, assuming relatively constant δ18Owater/salinity gradients since the time of occupation, allowing for an examination of shifts in oyster habitat exploitation over time. Our previous (Thompson et al. 2015) study indicated that midden accumulation occurred throughout the year, while oysters from mound deposits were collected in colder months. New data indicate that in addition to differential season of collection, habitat exploitation also varied. During early occupation at the site, oysters were collected primarily from lower saline habitats, while in later phases oysters were obtained from higher salinity waters; we relate this to a lower sea level and concomitant settlement shift seaward. Additionally, oyster from later mound contexts was collected from higher saline habitats relative to those in midden contexts; Native people may have targeted specific bioherms at certain times for the year for feasting-related mound construction.

Molluscs and Paleoenvironmental Reconstruction in Island and Coastal Settings: Variability, Seasonality, and Sampling

In recent years there have been significant methodological advances in sampling and interpreting oxygen isotope data from mollusc shells recovered from archaeological sites. In particular, the oxygen isotope record in mollusc shells derived from island and coastal sites reveals trends in mollusc collection intensity, seasonality, and paleoenvironmental conditions. These lines of evidence add a new dimension for interpreting human-environmental interactions. Sampling strategies that are appropriate for the growth rate of the species in question and the research objectives are essential for accurately interpreting oxygen isotope results. Therefore, we argue that high-resolution sampling methods provide more useful results than low-resolution methods, in both seasonality studies and paleoenvironmental reconstruction. This chapter will review current techniques in the geochemical analysis of mollusc shells, issues related to sampling methods and interpretation, and will address future directions for this method.