Cosmogenic nuclide exposure ages for moraines in the Lago San Martin Valley, Argentina

At several times during the Quaternary, a major eastward-flowing outlet glacier of the former Patagonian Ice Sheet occupied the Lago San Martin Valley in Argentina (49°S, 72°W). We present a glacial chronology for the valley based on geomorphological mapping and cosmogenic nuclide (¹⁰Be) exposure ages (n = 10) of boulders on moraines and lake shorelines. There are five prominent moraine belts in the Lago San Martin Valley, associated with extensive sandar (glaciofluvial outwash plains) and former lake shorelines. Cosmogenic nuclide exposure ages for boulders on these moraines indicate that they formed at 14.3 ± 1.7 ka, 22.4 ± 2.3 ka, 34.4 ± 3.4 ka to 37.6 ± 3.4 ka (and possibly 60 ± 3.5 ka), and 99 ± 11 ka (1σ). These dated glacier advances differ from published chronologies from the Lago San Martin Valley based on ¹⁴C age determinations from organic sediments and molluscs in meltwater channels directly in front of moraines or in kettleholes within end moraine ridges. The moraine boulder ages also point to possible pre-LGM glacial advances during the last glacial cycle and a key observation from our data is that the LGM glaciers were probably less extensive in the Lago San Martin Valley than previously thought.     

Glacial Lake Victoria,a high‐level Antarctic Lake inferred from lacustrine deposits in Victoria Valley

We present evidence of a large lake (Glacial Lake Victoria) that existed in Victoria Valley in the dry valleys region of Antarctica between at least 20 000 and 8600 ¹⁴C yr BP. At its highstands, Glacial Lake Victoria covered 100 km² and was ca. 200 m deep. The chronology for lake‐level changes comes from 87 AMS radiocarbon dates of lacustrine algae preserved in deltas and glaciolacustrine deposits that extend up to 185 m above present‐day lakes on the valley floor. The existence of Glacial Lake Victoria, as well as other large lakes in the dry valleys, indicates a climate regime significantly different from that of today at the last glacial maximum and in the early Holocene. Copyright © 2002 John Wiley & Sons, Ltd.     

The use of ‘bomb spike’ calibration and high-precision AMS C analyses to date salt-marsh sediments deposited during the past three centuries

A combination of ‘bomb spike’ calibration and conventional calibration of AMS ¹⁴C dating has been used to determine a detailed age-depth model for a 1-m sediment section collected from a salt marsh in Poole Harbour, southern England. These data were compared with the chronology obtained from ²¹⁰Pb analysis and ¹³⁷Cs age markers. We report post bomb values of over 1.46 F¹⁴C (> 146% modern ¹⁴C), and both the rising and falling limbs of the atmospheric ‘bomb spike’ are identified. Five pre-bomb samples were analysed using multi-target high-precision 2‰ AMS analysis, and after the replicates were combined the one-sigma uncertainty was as low as ± 9 ¹⁴C yr on some ages. These data, and an additional three normal-precision pre-bomb ¹⁴C samples, were calibrated using CALIB 5.0 and the chronology constrained using the ‘prior knowledge’ of independent age markers obtained from the analysis of pollen and spheroidal carbonaceous particle (SCPs). No agreement was found between the ¹⁴C ‘bomb spike’ dates and the CRS ²¹⁰Pb chronology modelled for this sequence. In addition, poor agreement was found between the signal of the 1960s weapons test fallout indicated by the ¹⁴C ‘bomb spike’ dates and the timing suggested by the ¹³⁷Cs data. This disagreement is attributed to the influence of the local discharge of ¹³⁷Cs from the former UKAEA site at Winfrith. We use our new chronology to confirm the existence of an acceleration in sedimentation rates in Poole Harbour during the last 100 yr previously reported for this site by Long et al. (Long, A.J., Scaife, R.G., Edwards, R.J. 1999. Pine Pollen in intertidal sediments from Poole Harbour, UK; implications for late-Holocene sediment accretion rates and sea-level rise. Quaternary International, 55, 3–16.), and conclude that ‘bomb spike’ ¹⁴C calibration dating may offer a more robust alternative to the use of ²¹⁰Pb chronologies for dating sediment deposition in salt-marsh environments. In addition, we demonstrate how the use of high-precision AMS analysis has the potential for reducing some of the uncertainties involved in the high-resolution dating of recent salt-marsh sediments.     

Timing and duration of North American glacial lake discharges and the Younger Dryas climate reversal

Radiocarbon-dated sediment cores from the Champlain Valley (northeastern USA) contain stratigraphic and micropaleontologic evidence for multiple, high-magnitude, freshwater discharges from North American proglacial lakes to the North Atlantic. Of particular interest are two large, closely spaced outflows that entered the North Atlantic Ocean via the St. Lawrence estuary about 13,200–12,900 cal yr BP, near the beginning of the Younger Dryas cold event. We estimate from varve chronology, sedimentation rates and proglacial lake volumes that the duration of the first outflow was less than 1 yr and its discharge was approximately 0.1 Sv (1 Sverdrup = 10⁶ m³ s⁻¹). The second outflow lasted about a century with a sustained discharge sufficient to keep the Champlain Sea relatively fresh for its duration. According to climate models, both outflows may have had sufficient discharge, duration and timing to affect meridional ocean circulation and climate. In this report we compare the proglacial lake discharge record in the Champlain and St. Lawrence valleys to paleoclimate records from Greenland Ice cores and Cariaco Basin and discuss the two-step nature of the inception of the Younger Dryas.     

Middle and late Pleistocene glaciations in the Campo Felice Basin (central Apennines, Italy)

The present paper refers to research conducted in the tectonic-karst depression of Campo Felice in the central Apennines (Italy), in which glacial, alluvial and lacustrine sediments have been preserved. Stratigraphic interpretations of sediments underlying the Campo Felice Plain are based on evidence obtained from nine continuous-core boreholes. The boreholes reach a depth of 120 m and provide evidence of five sedimentation cycles separated by erosion surfaces. Each cycle is interpreted as an initial response to a mainly warm stage, characterized by low-energy alluvial and colluvial deposition, pedogenesis, and limited episodes of marsh formation. In turn, a mainly cold stage follows during which a lake formed, and glaciers developed and expanded, leading to deposition of glacial and fluvioglacial deposits. The chronological framework is established by eleven accelerator mass spectrometer ¹⁴C ages and three ³⁹Ar-⁴⁰Ar ages on leucites from interbedded tephra layers. These age determinations indicate five glacial advances that respectively occurred during marine oxygen isotope stages 2, 3-4, 6, 10 and 14.     

The variations of stable-carbon isotope ratios in Qilian juniper in northwestern China

This is the first study of δ¹³C values that considers elevational influences of the Qilian mountains of China. The δ¹³C values of different parts of the Qilian juniper (Sabina przewalskii Kom.) at different elevations of the Qilian mountains, China, were measured. Relatively constant differences in δ¹³C values existed between the tree leaves and the trunk wood. No strong dependency of δ¹³C values on elevation was found. The temporal changes of wood δ¹³C values with directions, elevations and height in the tree showed general similarity. These results suggest that the isotopic effects of environmental factors expressed initially at the site of photosynthesis, i.e., the leaves could be contained in the trunks of a tree. Regardless of elevation, direction or height in the tree, wood δ¹³C values have been decreasing during the past 60 years. A composite 600-year δ¹³C chronology developed from four trees showed that δ¹³C values remain relatively constant before 1800. After 1800 AD, however, δ¹³C values became significantly more negative. The trend in wood δ¹³C values was consistent with that obtained for δ¹³C values in atmospheric CO₂. The results showed that for Qilian juniper, tree-ring δ¹³C chronology could be established to ascertain fluctuations of the atmospheric δ¹³C.     

Asynchroneity of maximum glacier advances in the central Spanish Pyrenees

The deglaciation history of the Escarra and Lana Mayor glaciers (Upper Gállego valley, central Spanish Pyrenees) had been reconstructed on the basis of detailed geomorphological studies of glacier deposits, sedimentological and palynological analyses of glacial lake sediments and an accelerator mass spectrometry (AMS) ¹⁴C chronology based on minimum ages from glacial lake deposits. The maximum extent of the Pyrenean glaciers during the last glaciation was before 30 000 yr BP and pre‐dated the maximum advances of the Scandinavian Ice Sheet and some Alpine glaciers. A later advance occurred during the coldest period (around 20 000 yr BP), synchronous with the maximum global ice extent, but in the Pyrenees it was less extensive than the previous one. Later, there were minor advances followed by a stage of debris‐covered glaciers and a phase of moraine formation near cirque backwalls. The deglaciation chronology of the Upper Gállego valley provides more examples of the general asynchroneity between mountain and continental glaciers. The asynchroneity of maximum advances may be explained by different regional responses to climatic forcing and by the southern latitude of the Pyrenees. Copyright © 2002 John Wiley & Sons, Ltd.     

A direct comparison of C and Th ages at Searles Lake, California

Thorium-230 dating on saline of the Lower Salt unit in pluvial Searles Lake, California, shows that this unit was formed between 24,500 and 32,000 years ago. The initial apparent ¹⁴C age of the lake is estimated to be about 900 years. After correcting for nonradiogenic ²³⁰Th and for the initial ¹⁴C age, excellent agreement between ²³⁰Th and ¹⁴C ages is obtained. The reliability of ²³⁰Th dating on salt deposit opens a new way for continuation of absolute chronology below the Lower Salt in Searles Lake.     

A direct comparison of 14C and 230Th ages at Searles Lake,California

Thorium-230 dating on saline of the Lower Salt unit in pluvial Searles Lake, California, shows that this unit was formed between 24,500 and 32,000 years ago. The initial apparent ¹⁴C age of the lake is estimated to be about 900 years. After correcting for nonradiogenic ²³⁰Th and for the initial ¹⁴C age, excellent agreement between ²³⁰Th and ¹⁴C ages is obtained. The reliability of ²³⁰Th dating on salt deposit opens a new way for continuation of absolute chronology below the Lower Salt in Searles Lake.     

Uplift and active tectonics of southern Albania inferred from incision of alluvial terraces

In Albania, the Osum and Vjoje rivers cross the active graben system and the active frontal thrust system of the Albanides. The effects of climatic and geodynamic forcing on the development of these two rivers were investigated by the means of field mapping, topographic surveying and absolute exposure-age dating. We established the chronology of terraces abandonment from the compilation of new dating (¹⁴C and in situ produced ¹⁰Be) and previously published data. We identified nine fluvial terraces units developed since Marine Isotope Stage 6 up to historic times. From this reconstituted history, we quantified the vertical uplift on a time scale shorter than the glacial climatic cycle. Regional bulging produces an overall increase of the incision rate from the west to the east that reaches a maximum value of 2.8 m/ka in the hinterland. Local pulses of incision are generated by activation of normal faults. The most active faults have a SW–NE trend and a vertical slip rate ranging from 1.8 to 2.2 m/ka. This study outlines the geodynamic control in the development of rivers flowing through the Albanides on the scale of 10³–10⁵ka.     

Associated terrestrial and marine fossils in the late-glacial Presumpscot Formation, southern Maine, USA, and the marine reservoir effect on radiocarbon ages

Excavations in the late-glacial Presumpscot Formation at Portland, Maine, uncovered tree remains and other terrestrial organics associated with marine invertebrate shells in a landslide deposit. Buds of Populus balsamifera (balsam poplar) occurred with twigs of Picea glauca (white spruce) in the Presumpscot clay. Tree rings in Picea logs indicate that the trees all died during winter dormancy in the same year. Ring widths show patterns of variation indicating responses to environmental changes. Fossil mosses and insects represent a variety of species and wet to dry microsites. The late-glacial environment at the site was similar to that of today's Maine coast. Radiocarbon ages of 14 tree samples are 11,907 ± 31 to 11,650 ± 50 ¹⁴C yr BP. Wiggle matching of dated tree-ring segments to radiocarbon calibration data sets dates the landslide occurrence at ca. 13,520 + 95/−20 cal yr BP. Ages of shells juxtaposed with the logs are 12,850 ± 65 ¹⁴C yr BP (Mytilus edulis) and 12,800 ± 55 ¹⁴C yr BP (Balanus sp.), indicating a marine reservoir age of about 1000 yr. Using this value to correct previously published radiocarbon ages reduces the discrepancy between the Maine deglaciation chronology and the varve-based chronology elsewhere in New England.     

Vegetation and climate near Lago Llanquihue in the Chilean Lake District between 20200 and 9500 14C yr BP

Radiocarbon-dated pollen records of two adjacent sediment cores from Canal de la Puntilla (40°57′09″S, 72°54′18″W) in the Chilean Lake District reveal that a sparsely vegetated landscape prevailed during the portion of the Last Glacial Maximum between 20200 and about 14800 ¹⁴C yr BP. Dominating the vegetation was Nothofagus, Gramineae and Compositae, along with taxa commonly found today above the Andean treeline (Perezia-type, Valeriana) and in Magellanic Moorlands (Donatia, Astelia). Nothofagus expanded between 20200 and 15800 ¹⁴C yr BP, interrupted by a reversal at 19200 ¹⁴C yr BP and followed by a prominent increase in Gramineae pollen between 15800 and about 14800 ¹⁴C yr BP. A major increase in Nothofagus started at about 14800 ¹⁴C yr BP, followed by an abrupt expansion of thermophilous Valdivian/North Patagonian Rain Forest taxa (Myrtaceae, Lomatia/Gevuina, Hydrangea, etc.) at about 14000 ¹⁴C yr BP. An opening of the rain forest and an expansion of Podocarpus nubigena, Misodendrum, and Maytenus disticha-type subsequently occurred between 11000 and 10000 ¹⁴C yr BP. These results suggest that mean annual temperature was 6–7°C colder than at present, with twice the modern annual precipitation between 20200 and 14000 ¹⁴C yr BP, implying a northward shift and intensification of the westerlies storm-tracks. Slight climate warming occurred between 20200 and 15800 ¹⁴C yr BP, featuring cooling reversals at 19200 ¹⁴C yr BP, and later at 15800 ¹⁴C yr BP. The warming of the last termination started at about 14800 ¹⁴C yr BP, and reached a total temperature rise of ≥5°C by 12400 ¹⁴C yr BP, followed by cooling between 11000 and 10000 ¹⁴C yr BP. © 1997 John Wiley & Sons, Ltd.     

Early Holocene environmental history of sunken landscapes: pollen, plant macrofossil and geochemical analyses from the Borkum Riffgrund, southern North Sea

A vibrocore from the sea floor of the southern North Sea provides a ~1,500-year record of early Holocene vegetation history and mire development in a landscape now 33 m below sea-level. Pollen, plant macrofossil and geochemical analyses of an AMS ¹⁴C dated sand–peat–marine mud sequence document the paludification on Pleistocene sands ~10,700 cal BP, the subsequent development of eutraphentic carr vegetation and the gradual inundation by the transgressing sea ~9,350 cal BP. Pinus–Corylus woodland prevailed on terrestrial grounds after hazel had immigrated ~10,700 cal BP. Salix dominated the carr vegetation throughout 1,300 years of peat formation, because Alnus did not spread in the Borkum Riffgrund area until 9,300 BP. Brackish reed vegetation with Phragmites established after inundation and siliciclastic marine sediments were being deposited. This article also examines the detection and suitability of key horizons indicative of marine influence. XRF-Scanning provides the most detailed results in the briefest possible time to pinpoint spectra best suitable for AMS ¹⁴C dating of classical key horizons such as start of peat formation and transgressive contact. The combined application of botanical and geochemical methods allows determining new key horizons indicative of marine influence, namely the earliest marine inundation and the onset of sea-level influence on coastal ground water level.     

Revised marine reservoir offset (ΔR) values for molluscs and marine mammals from Arctic North America

Appropriate marine–terrestrial reservoir offset (ΔR) values are essential for accurate calibration of marine radiocarbon dates. However, ΔR values are only valid for the specific calibration curve that their calculation is based on. Here, we present revised ΔR values for the Marine20 calibration curve from Arctic North America, based on previously published ¹⁴C dates on pre-bomb live-collected marine molluscs (n = 124) and cetaceans (beluga whales; tooth dentine; n = 12), and bowhead whale–driftwood age comparisons from the same glacio-isostatically uplifted shorelines (n = 18). Molluscan-based ΔR are: Chukchi/Beaufort sea coasts, 265±116 ¹⁴C years; NW Canadian Arctic Archipelago, 188±91 ¹⁴C years; NE Baffin Island, 81±18 ¹⁴C years; SE Baffin Island, 14±58 ¹⁴C years; Hudson Strait, −73±64 ¹⁴C years; Ungava Bay, 0±86 ¹⁴C years; Foxe Basin, 175±89 ¹⁴C years; Hudson Bay, −21±72 ¹⁴C years; James Bay, 209±114 ¹⁴C years; West Greenland, −93±111 ¹⁴C years. Species-specific marine mammal ΔR terms are 107±59 ¹⁴C years for beluga and 24±58 ¹⁴C years for bowheads. Our revised ΔR values are applicable for as long as the same broad oceanographic conditions (circulation, ventilation) have persisted, i.e. through the Holocene. While molluscan values are applicable to other marine carbonate (e.g. foraminifera), cetacean ΔR are valid only for the species they were calculated for and should not be applied to other marine mammals. Importantly, the ΔR terms calculated here are only valid for Marine20 and should not be used with earlier or later calibration curves.     

Holocene climate phases from buried soils in Tigray (northern Ethiopia): comparison with lake level fluctuations in the Main Ethiopian Rift

Stratigraphic analysis of alluvial/colluvial sequences and ¹⁴C dating have been used as proxies for Holocene climate changes in the highlands of Tigray (northern Ethiopia). The studied records show alternations of buried soils and peaty–clayey sediments, pointing to wet, stabilization phases, and organic-free colluvium layers resulting from the abrupt occurrence of dry-climate episodes. The ¹⁴C dates, mostly unpublished, cluster in the 11,090–9915, 9465–9135, 8450–7330, 6720–3635, 2710–2345, and 1265–790 cal yr B.P. time spans. Evidence of subsequent pedogenesis is lacking in the area, apart from a buried humified horizon dated at 300 ± 60 ¹⁴C yr B.P. (460–295 cal yr B.P.). Both the timing and the pattern of Tigray paleoclimatic events fit the corresponding framework, based on lake level changes, previously implemented for the Main Rift Valley. These findings give further support for arguing that the forcing mechanisms of the wet/dry fluctuations during the Holocene were effective over a large scale.     

Late-Glacial and Early Holocene Environmental and Climatic Change at Lake Tambichozero, Southeastern Russian Karelia

High-resolution lithostratigraphy, mineral magnetic, carbon, pollen, and macrofossil analyses, and accelerator mass spectrometry ¹⁴C measurements were performed in the study of a sediment sequence from Lake Tambichozero, southeastern Russian Karelia, to reconstruct late-glacial and early Holocene aquatic and terrestrial environmental changes. The lake formed ca. 14,000 cal yr B.P. and the area around the lake was subsequently colonized by arctic plants, forming patches of pioneer communities surrounded by areas of exposed soil. A minor rise in lake productivity and the immigration of Betula pubescens occurred ca. 11,500 cal yr B.P. The rise in summer temperatures probably led to increased melting of remnant ice and enhanced erosion. The distinct increase in lake productivity and the development of open Betula-Populus forests, which are reconstructed based on plant macrofossil remains, indicate stable soils from 10,600 cal yr B.P. onward. Pinus and Picea probably became established ca. 9900 cal yr B.P.     

Changes to depositional palaeoenvironments within the Qikou depression (Bohaiwan Basin,China): carbon and oxygen isotopes in lacustrine carbonates of the Palaeogene Shahejie Formation

This paper reports the carbon and oxygen isotope compositions of lacustrine carbonate sediments from the Palaeogene Shahejie Formation, Qikou depression, Bohaiwan Basin, with the aim of determining the palaeoenvironmental conditions in the region. Results from Es₂, the second member of the Shahejie Formation, showed values of δ¹³C and δ¹⁸O from –1.2‰ to +2.4‰ (average +0.6‰) and from –6.8‰ to –4.7‰ (average –5.7‰), respectively, suggesting a relatively hot climate attending deposition. The slightly closed nature of the lake, which contains brackish water, resulted in higher carbonate δ¹³C and δ¹⁸O values than in a meteoric environment. The values of δ¹³C and δ¹⁸O preserved within the carbonates of the overlying lower Shahejie I (Es₁) varied between +1.3‰ and +4.9‰ (average +3.2‰) and from ?4.4‰ to ?1.8‰ (average ?3.1‰), respectively, indicating that the climate became colder at that time. Subsequently, a marine transgression caused the salinity of the lake water to increase. The values of δ¹³C and δ¹⁸O were controlled by salinity. The high δ¹³C values were also influenced by the rapid burial of the lake organisms and by algal photosynthesis. Values of δ¹³C and δ¹⁸O from carbonates in upper Es₁ ranged from ?8.0‰ to +11.0‰ (average +10.1‰) and from ?5.0‰ to ?1.5‰ (average ?3.4‰), respectively, indicating a slight increase in the temperature over time. In the closed and reducing environment, extremes in δ¹³C values resulted from biochemical fermentation. The positive δ¹³C excursion recorded in the carbonates of the Shahejie Formation in the Qikou depression indicates that the palaeoclimate underwent a significant transformation during the Eocene and the Oligocene.     

A 16 000‐year record of vegetational change in south‐western Alaska as inferred from plant macrofossils and pollen

Pollen and macrofossil analyses of a sediment core from Beaver Pond (60° 37′ 14″ N, 154° 19′ W, 579 m a.s.l.) reveal a record of regional and local postglacial vegetation change in south‐western Alaska. The chronology is based on five AMS (accelerator mass spectrometry) ¹⁴C ages obtained from terrestrial plant macrofossils. Pollen and macrofossil records suggest that open herb and shrub tundra with e.g. Poaceae, Cyperaceae, Artemisia, Vaccinium and Salix prevailed on the landscape before ca. 14 000 cal a BP. The shift from herb‐ to shrub‐dominated tundra (Salix, subsequent Betula expansion) possibly reflects climatic warming at the beginning of the Bølling period at ca. 14 700–14 500 and around 13 500 cal a BP. Vegetation (Betula shrub tundra) remained relatively stable until the early Holocene. Macrofossil influx estimates provide evidence for greater biomass in Betula shrub tundra during the early postglacial period than today. Charcoal accumulation rates suggest tundra fire activity was probably greater from ca. 12 500 to 10 500 cal a BP, similar to results from elsewhere in Alaska. The pollen and macrofossil records of Beaver Pond suggest the prevalence of low shrub tundra (shrub Betula, Betula nana, Vaccinium, Ledum palustre, Ericaceae) and tall shrub tundra (Alnus viridis ssp. crispa, Salix) between 10 000 and 4000 cal a BP. This Holocene vegetation type is comparable with that of the modern treeless wet and moist tundra in south‐western Alaska. The expansion of Picea glauca occurred ～4000 cal a BP, much later than that of A. viridis (ssp. crispa), whereas in central and eastern Alaska Picea glauca expanded prior to or coincident with Alnus (viridis). At sites located only 200–400 km north‐east of Beaver Pond (Farewell and Wien lakes), Picea glauca and Betula forests expanded 8000–6000 cal a BP. Unfavourable climatic conditions and soil properties may have inhibited the expansion and establishment of Picea across south‐west Alaska during the mid and late Holocene. Copyright © 2011 John Wiley & Sons, Ltd.     

Suitability of biogenic carbonate of Lithospermum fruits for C dating

Lithospermum (Boraginaceae) belongs to a small group of plant taxa that accumulate biogenic carbonate in their fruits. In this genus, carbonate incrustations form in the cells of the epidermis and sclerenchyma of the pericarp. Fossil Lithospermum fruits (nutlets) with well-preserved calcified tissues commonly occur in Quaternary sediments and cultural layers. We tested the suitability of biogenic carbonate of Lithospermum fruits for radiocarbon dating using a total of 15 AMS measurement results from four modern and 11 fossil samples. The ¹⁴C data from modern samples suggest that Lithospermum utilises only atmospheric carbon to synthesise calcite in the nutlets. In general, the ages determined through ¹⁴C dating of fossil fruitscorresponded well with the absolute-age intervals for archaeological sites over the last 5000 yr. Biogenic carbonate of Lithospermum fruits, like that of Celtis, represents a new source of chronological information for late Quaternary studies.     

Upwelling signals in radiocarbon from early 20th-century Peruvian bay scallop (Argopecten purpuratus) shells

We quantified Δ¹⁴C, δ¹⁸O, and δ¹³C cycles along ontogeny within four bay scallop (Argopecten purpuratus) shells collected from Callao Bay, Salaverry, and Sechura Bay, Peru following the 1907–1908 non-El Niño years and the 1925–1926 El Niño. Δ¹⁴C and δ¹³C generally covary; Δ¹⁴C and δ¹⁸O vary inversely. Simultaneous decreases in Δ¹⁴C and increases in δ¹⁸O in non-El Niño shells are followed by constant Δ¹⁴C and gradually decreasing δ¹⁸O, which we interpret as evidence for discrete marine upwelling events followed by warming of the initially cold upwelled water. Upwelling changes from El Niño events are detectable with difficulty in mollusk shell Δ¹⁴C.