Investigaciones Relacionadas

Isotopic compositions obtained for “bulk” analyses are means that average growth over several seasons, as are the mean δ-values of all incremental samples from a given specimen. Are these multi-season means directly comparable? There is a good correlation between bulk and incremental means for both δ13C (r = 0.92, df = 4, p<0.01) and δ18O (r = 0.97, df = 4, p<0.01). There is no systematic offset between incrementally sampled

142 mean δ13C values and bulk δ13C values (mean difference = -0.2‰, range of offsets = -1.3 to 0.6‰), but the incrementally sampled mean δ18O values are on average 1.8‰ lower than the bulk values (range of offsets = 2.5 to 1.0‰ lower). These differences cannot be attributed to the fact that incremental samples were not pretreated, because (1) three of the bulk specimens were pretreated and three were not, yet all showed offsets, (2) the offsets were larger than those observed as a result of pretreatment (see Section 5.2.2), and (3) the incremental-bulk difference was slightly smaller for pretreated than untreated bulk samples, even though pretreatment should have increased the offset by increasing the bulk δ18O values (see Section 5.2.2).

There are several possible explanations for the lower mean δ18O values obtained for incremental samples relative to “bulk” samples: dentin contamination of incrementally sampled enamel, a seasonal bias in either of the means, or differences in mineralization processes. The first possibility can be rejected because dentin δ18_{O values (between 19.7}

and 20.1‰, measured for two different individuals) are higher than the incrementally- sampled mean δ18O value for one of the specimens. A seasonal bias can also be rejected since one specimen displays no “seasonal” variations in oxygen isotope composition and yet still shows the typical offset. Also, the δ13C curves tend to show greater seasonal variation than the δ18O curves for most samples, yet there was no pretreatment offset for δ13C values. A difference in mineralization processes is arguably the most likely

explanation, since the innermost enamel (i.e., that used for incremental sampling)

mineralizes faster than the rest of the enamel in various species (Allan, 1967; Tafforeau et al., 2007). More rapid mineralization could involve differences in the isotopic

composition of fluids from which bioapatite forms, as a result of changes in the transport of bicarbonate ions to developing enamel by ameloblasts (Smith, 1998), or of depletion of bicarbonate in the fluid surrounding the enamel, which would result in the incorporation of more 16O-containing bicarbonate into bioapatite. Alternatively, the rapid formation of inner enamel could be insufficient for bioapatite to reach isotopic equilibrium with enamel fluids. Regardless of the explanation, the consistent offset between “bulk” and mean “incremental” δ18O values indicates that the values obtained using the different methods are not directly comparable. However, the strong correlations between the

5.4.2 Bulk Carbonate

The mean δ18Owater value calculated using Clovis-age SPV mammoths (-6.6 ±

1.1‰) isvery close to the modern amount-weighted mean δ18O of San Pedro Valley precipitation during the summer (-6.2‰) (Wahi et al., 2008), and higher than modern SPV groundwater, river water, or monsoon floodwater (see Section 5.1.2). The similarity to modern summer precipitation δ18O values does not necessarily indicate that the

Terminal Pleistocene climate was as warm and dry as that of the modern San Pedro Valley, although numerous lines of evidence do indicate that it was warm and dry (Haynes, 2005; Holmgren et al., 2006; Pigati et al., 2009; Wagner et al., 2010). Higher δ18O values can also result from (1) increased evaporation, (2) differences in moisture source and atmospheric circulation patterns (i.e., rain clouds that originated closer to the San Pedro Valley, evaporated from cooler surface water with lower specific humidity, and experienced less rainout prior to reaching the SPV), (3) greater input from summer than winter precipitation, and (4) greater input from lower- than higher-altitude

precipitation. Any or all of these factors could have contributed to the relatively high

δ18Owater values.

The δ18Owater values calculated using the pre-Clovis mammoths (Table 5.2)are

within the range of modern winter precipitation δ18O values from Tucson and the San Pedro Valley (Baillie et al., 2007; Wagner et al., 2010, supplementary information). These lower values could theoretically result from either behavioural differences (i.e., selection of different water sources, migration to different geographical regions) or climatic differences (i.e., temperature, rainfall amount, contributions of winter vs.

summer precipitation, moisture source region, atmospheric circulation patterns). Since the two specimens with the lowest δ18O values (Murray Springs, Moson Wash) were

recovered from older contexts than the rest of the samples, a climatic explanation is likely at least partially responsible. The lower δ18O values of the pre-LGM individuals suggest cooler and/or wetter climates. The pre-LGM mammoths also consumed substantially fewer C4 plants, which tend to grow under warmer conditions. These interpretations are

consistent with the multiple lines of evidence that show generally cooler and wetter conditions prevailed in the American Southwest during much of the Pleistocene (Thompson et al., 1993).

144

5.4.3 Incremental Carbonate

5.4.3.1 Clovis Mammoths

Considering that sinusoidal variations in the δ13C and δ18O valuesof tooth enamel are characteristic of seasonal changes in drinking water and diet, and that growth rate estimates from a thin section of Eloise are consistent with estimates based on the distance between isotopic maxima or minima in these curves (Chapter 4), and that periods for δ13C curves among individuals are similar (Table 5.4) we assume that one period of variation represents one year of growth for Clovis mammoths. Since the δ18O values of meteoric water (and river, lake, or groundwater with meteoric water inputs) are higher in summer and lower in winter (Baillie et al., 2007; Wahi et al., 2008), the peaks in the δ18O and δ13C curves in Clovis mammoths (which approximately coincide) almost certainly represent summer, and the troughs, winter.

The correspondence between peaks and troughs in the δ13C and δ18O curvesof Clovis mammoths suggest that they were eating C4-dominated diets in summer, and

mixed C3-C4 diets in winter. Summer highs ranged from 62-83% C4 plants, while winter

lows were between 4 and 50% C4 plants (Tables 5.3, 5.5). Since the same pattern of

seasonal dietary change was observed in enamel from several individuals, and mammoth tooth enamel grows over a period of years to decades (Chapter 3) (Metcalfe et al., 2010), we infer that Clovis mammoth seasonal dietary behaviour was quite consistent over time (Figure 5.5). The amplitudes of variation in δ18O values for Clovis-age mammoths were similar to those of modern seasonal precipitation in the SPV (Figure 5.6). Bearing in mind that the former represent minimum amplitudes of drinking water variations (because of “dampening” that can occur as a result of enamel maturation and sampling geometry; see Chapter 4), this suggests that seasonal variations in Pleistocene SPV drinking water were at least as great as those of modern SPV precipitation.

The Horsethief Draw (AZ13) mammoth’s incrementally sampled δ18O pattern differs from those of the other Clovis-age mammoths (Figure 5.4). We infer that for this

Table 5.5 Summary of stable isotope results for "incremental" analyses, and corresponding diet and drinking water compositions.

δ13_C

sc (‰) %C4 δ18Osc (‰) δ18Owater (‰)

LSIS # Site n Mean Min Max Range Mean1 Mean2 Min3 Max3 Mean Min Max Range Mean Min Max Range

Clovis-associated and Clovis-age

AZ1 (Eloise) Murray Springs 38 -2.2 ± 1.4 -5.4 -0.3 5.2 62 ±10 43 ± 15 9 - 39 62 - 75 23.1 ± 0.7 21.8 23.1 2.3 -9.6 ± 0.7 -11 -8.5 2.4 AZ5 Lehner 25 -1.1 ± 1.3 -4.5 0.4 4.8 69 ± 9 54 ± 13 19 - 46 69 - 79 25.9 ± 1.3 23.8 28.2 4.3 -6.6 ± 1.4 -8.8 -4.3 4.5 AZ10 Naco 43 -2.2 ± 1.8 -5.9 -0.3 5.6 61 ± 12 42 ± 18 4 - 36 62 - 75 24.0 ± 1.3 22.1 27.0 4.9 -8.6 ± 1.4 -11 -5.5 5.1 AZ13 HTD Loc 129 51 -0.9 ± 1.3 -3.8 0.9 4.7 70 ± 9 56 ± 13 26 - 50 74 - 83 23.1 ± 1.3 20.8 26.0 5.3 -9.6 ± 1.4 -12.0 -6.5 5.5

Pre-LGM (>29,000 BP)

AZ11 Murray Springs 41 -4.3 ± 0.9 -6.0 -2.6 3.4 47 ± 6 20 ± 9 3 - 35 38 - 59 19.6 ± 0.9 17.8 21.4 3.6 -13.2 ± 1.0 -15 -11 3.7 AZ14 Moson Wash 43 -5.1 ± 2.2 -9.0 -1.4 7.6 41 ± 15 12 ± 22 -28 - 14 51 - 67 22.3 ± 0.5 21.1 23.1 2.0 -10.4 ± 0.5 -12 -9.5 2.1 1 _%C

4 calculated using model 1, (lower δ13C value for C3 plant endmember - see text) 2 _%C

4 calculated using model 2, (higher δ13C value for C3 plant endmember, very conservative %C4 estimate - see text) 3 _{Minimum (maximum) %C}

4 range is calculated from the lowest (highest) δ13C values, using model 2 (first number) and model 1 (second number)

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 time (years) δ 13C ( ‰ , V P DB) AZ1 (Elois e) AZ10 (Naco) AZ5 (Lehner) AZ13 (HTD Loc129) 17 18 19 20 21 22 23 24 25 26 27 28 29 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 time (years) δ 18O ( ‰ , VSM O W ) AZ1 (Eloise) AZ10 (Naco) AZ5 (Lehner) AZ13 (HTD Loc129)

a. Clovis age, carbon c. Pre-LGM, carbon

b. Clovis age, oxygen d. Pre-LGM, oxygen

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 time (years) δ 13C ( ‰ , VP DB) AZ11E (MS) AZ14E (Moson) 17 18 19 20 21 22 23 24 25 26 27 28 29 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 time (years) δ 18O ( ‰ , V S M O W ) AZ11 (MS) AZ14 (Mos on)

Figure 5.5 Comparison of carbon and oxygen isotope results for “incremental” sampling of Clovis-age (a, b) and pre-LGM (c, d) mammoths from the SPV. The “distance” axis has been transformed to time, as described in the text. The reference point for time is arbitrary, so although the data are stacked for comparison of seasonal trends, the true “calendar” years of formation for each sample likely differ.

-18 -16 -14 -12 -10 -8 -6 -4 -2 δ 18 O wa te r ( ‰ , V S M O W ) Eloise Naco Lehner HTD MS MW GW RW ppt ppt pre-LGM SPV Tucson Mammoth Clovis summer winter

Figure 5.6 Comparison of seasonal extremes in the δ18O values of mammoth drinking water to the δ18O values of seasonal precipitation in the modern SPV and Tucson, and the δ18O values of modern SPV perennial water sources. Winter means are dark grey symbols and summer means are light grey. Mammoth seasonal extremes are the averages of peak (for summer) or trough (for winter) drinking water δ18O values (calculated from enamel δ18_{O values, as described in text) from each specimen's time-series curve. Thus, all data}

points are averages over several years/seasons. SPV water data are from Baillie (2005), Baillie et al. (2007); Tucson water data is from Wagner et al. (2010, supplementary material). Sampling geometry, enamel maturation, and residence time in the body results in some dampening of the seasonal extremes relative to those of drinking water. GW = groundwater, RW = river water, ppt = precipitation, SPV = San Pedro Valley, pre-LGM = pre-Last Glacial Maximum, MS = Murray Springs, MW = Moson Wash.

individual, as for the other Clovis mammoths, one period in the δ13C curve is equal to one year of growth. This inference is based on the similar period (19-22 mm) and shape of the HTD δ13_{C curve to those of the Naco mammoth (period = 23 mm), both of which were}

obtained from developing enamel cones and thus should have had similar growth rates (Table 5.4). Unlike the other Clovis-age mammoths, the HTD mammoth has large and sustained decreases in δ18O values during the latter half of the inferred summers (Figure 5.4). This indicates either very different drinking water sources during late summer, unusual metabolic effects, or significantly different meteorological conditions. The similarity of the δ13C curves of the HTD and other Clovis-age mammoths suggests that their seasonal dietary patterns were not considerably different. We know of no metabolic

148 effect that would decrease δ18O values by 3-5‰ during late summer but leave the rest of the seasonal pattern unaffected. However, an “amount effect” resulting from severe, sustained monsoon rains during the latter half of the summer could produce large decreases in summer precipitation δ18O values. This pattern is seen in modern

precipitation at Waco, Texas during early fall (September-October): temperatures remain high but the amount of precipitation more than doubles relative to the previous months, and the δ18O values decrease by more than 5‰ (Higgins and MacFadden, 2004;

IAEA/WMO, 2010). In November, less rainfall occurs and the δ18O values of

precipitation increase. Monsoon rainfall would also explain the slight increase in δ13C values that coincides with the low δ18O values, and the generally higher δ13C values of this individual relative to the other Clovis-age mammoths (i.e., more “greening up” of seasonal C4 plants).

5.4.3.2 Pre-LGM Mammoths

The patterns of the pre-LGM mammoth “incremental” results are very different from those of Clovis mammoths, and also from one another (Figures 5.4, 5.5). Given that the radiocarbon ages of these mammoths could differ by thousands of years, this is not unexpected.

AZ11 (Murray Springs) has smooth, sinusoidal variations in δ18O values, and a period (>9-16 mm) similar to those of the Clovis-age mammoths (Figure 5.4, Table 5.4). However, all the δ18O values of this mammoth, including the highest peaks in the time- series, are lower than those of the Clovis mammoths (Figure 5.5), suggesting lower temperatures and/or greater rainfall amounts throughout the entire year. Thus, this individual likely lived in the SPV during a glacial or stadial period. The relatively small range of δ18O values, and smooth appearance of the curve, suggests less seasonal

variability in meteoric water δ18O values, which further suggest wetter conditions, smaller seasonal temperature or humidity variations, and/or reliance on a more stable water source. The δ13C values are generally lower than those of the Clovis-age mammoths, indicating less C4 plant consumption, but there is still good evidence for some C4 plant

consumption at various times of the year (enamel δ13C values greater than -7‰) (Cerling et al., 1997), consistent with a generally cooler climate. The magnitude of variation in

δ13C values for AZ11, while small, is greater than those typical of mammoths living in C3-only ecosystems (Chapters 6, 7), which further suggests mixed C3-C4 plant

consumption. However, unlike the single peak in C4 plant consumption during summer

observed in the Clovis-age mammoths, this individual had 2 to 3 peaks in C4-plant

consumption per year, perhaps as a result of greater movement among microhabitats (grasslands, woodlands) throughout the year.

AZ14 (Moson Wash) has unique δ13C and δ18O patterns. The range of δ18O values is very small (2.0‰), and there is no apparent seasonal signal (Figure 5.4, Table 5.4). This likely resulted from reliance on a primary water source (or sources) with relatively invariant δ18O values (e.g., one of the large lakes that occupied playa basins in the southwest during much of the Pleistocene, or a groundwater spring). The δ18O values of this individual are higher than those of the other pre-LGM mammoth, and overlap the lower values of the Clovis-age mammoths, suggesting that it lived during a relatively warm interglacial period. The range of δ13_{C values for this individual (7.6‰) is much}

larger than that of any other SPV mammoth. The δ13C values for this individual indicate that maximum C4 plant consumption was intermediate between that of the Clovis

mammoths and the other pre-LGM mammoth. Minimum δ13C values were much lower than those of the Clovis mammoths or the other pre-LGM mammoth, indicating greater reliance on C3 plants or aquatic vegetation. In fact, most of the δ13C minima for this

mammoth are below the cutoff for unambiguous C4 plant consumption (Cerling et al.,

1997), suggesting 100% C3 plant consumption. Assuming that the growth rate of this

tooth was not extremely different from the other mammoth teeth (13-20 mm per year for mid-crown to cervical samples) (Table 5.4), we infer that peaks in C4 plant consumption

occurred 2-3 times per year, as was the case for the other pre-LGM mammoth (Figure 5.5). We speculate that this mammoth spent much of its time near large pluvial lakes, where C3 plants likely predominated, but periodically migrated to grassy areas further

from the lake, which supported C4 plants.