Evidence of Holocene climatic changes from Aeolian deposits in Baja California Sur, México

Quaternary International 56 (1999) 141—154

Evidence of Holocene climatic changes from Aeolian deposits in Baja California Sur, Me´xico Janette M. Murillo De Nava 
, Donn S. Gorsline , Glenn A. Goodfriend, V.K. Vlasov, Rodolfo Cruz-Orozco Department of Earth Sciences, University of Southern California, Univ. Park, LA, CA, 90089-0740, U.S.A.
Department of Oceanology, Centro Interdisciplinario de Ciencias Marinas-IPN. Apartado Postal 592, La Paz, B.C.S. CP 23000, Me& xico  Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015-1305, U.S.A.  Department of Chemistry and Geography, M. Lomonosov Moscow State University, The Lenin Hills, Moscow 119899, U.S.S.R.  Department of Geology, Universidad Auto& noma de Baja California Sur, Apdo. Postal 219 B, La Paz, B.C.S., Me& xico

Abstract Well-defined Late Pleistocene and Holocene dune fields are present on the surface of the Purı´ sima-Iray Magdalena sedimentary basin and on top of barrier islands of the Magdalena Lagoonal Complex in Baja California Sur, Me´xico. Thermoluminescence, amino acid epimerization, and radiocarbon techniques were used for dating dune sands and shell samples respectively. During Late Pleistocene to early Holocene (14 to 8 ka BP) mega-barchans and linear dunes were formed. During early Holocene through middle Holocene (warm-dry climate) dune fields were still active. At about 5 to 6 ka BP the stabilization of sea level developed the present lagoonal system in the area. Sandy barrier islands capped by foredunes were developed. At this time coastal dunes were eroded, and in part inundated, forming large flooding flats and sabkha deposits in the old inter-dune areas. During late Holocene, after the lagoonal system was fully developed, dunes were emplaced along the lagoonal coasts. The presence of some younger dates in older deposits seems to represent dune reactivation.  1999 INQUA/Elsevier Science Ltd. All rights reserved.

1. Introduction The surface of the central portion of the Purı´ sima-Iray Magdalena basin, exhibits low and high relief areas. Sedimentary rocks, fluvial, aeolian and coastal deposits are present. The old dune morphology shows well-defined mega-barchans, linear dunes and dune ridges, which have been eroded in part and covered by semi-arid vegetation. Recent dunes include transverse dunes, barchans, nabkhas, and foredunes. The chronology of these deposits was determined by dating mollusk shells found within aeolian deposits with the amino acid racemization method (AAR) and radiocarbon techniques (C), and by dating dune quartz sand with the thermoluminescence technique (TL). The basement of the aeolian deposits is in part caliche and in part alluvial material. Holocene dune emplacement in the study area shows depositional and nondepositional time intervals which can be associated with global climatic changes. Studies from different sedimentary environments show that inter* Corresponding author: E-mail: [email protected]

fingering of peat layers in rivers, and the development of soils are related to humid events (Chatters and Hoover, 1992; Salvador and Bravard, 1995); saline horizons and lake level changes (Enzel et al., 1996), presence of aeolian dust in glaciers, aeolian deposits in river terraces, and dune deposition world-wide, are related to warmer events (Muhs et al., 1990; Lees et al., 1990; Petit-Maire, 1995). This paper presents a chronology of Holocene arid events in Baja California Sur Mexico. In particular, dune reactivation, reworking and/or redeposition in old dune fields, evidence for an old coastline (possibly of Oxygen Isotope Substage 5e age) are examined. In addition, evidence that the Holocene sea level maximum was above the present sea level, and a review of the probable coastal evolution of the Magdalena Lagoonal Complex at the Holocene sea level stabilization is presented.

2. Study area and regional setting The study area is located in the central part of the Purı´ sima-Iray Magdalena Basin (Fig. 1). Purı´ sima-Iray

1040-6182/99/$20.00  1999 INQUA/Elsevier Science Ltd. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 9 8 ) 0 0 0 3 5 - 4

142

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

Fig. 1. Study area location, bathymetry of the continental shelf, and drainage basins on the western margin of the central portion of the Purı´ sima-Iray Magdalena basin, Baja California Sur, Me´xico.

Magdalena basin is one of the Baja California physiographic Provinces (Raisz, 1964). It is located on the western coast of the Baja California peninsula, Me´xico, between latitudes 26°00, and 24°15N and longitudes 112°15, and 111°00W. This basin is part of the Magdalena Lagoonal Complex, with an approximate area of 7850 km. The basin extends approximately 250 km along the coast. From the coast at 24°47N, the basin extends inland 90 km becoming narrower to the northwest and southwest. Landforms of the Purı´ sima-Iray Magdalena basin include scarps, hills, valleys, terraces and coastal dunes. To the northeast the morphology includes high and steep mountains of the La Giganta physiographic province. The stratigraphic sequence of the basin includes Paleocene to Pleistocene marine and vulcanosedimentary rocks, Pleistocene fluvial deposits, and Holocene aeolian deposits at the top (Raisz, 1964), fluvial-aeolian material and caliche (a thin, compact layer of flat blocks, boulders, cobbles and pebbles) are found as a basement in many areas of the dune fields, and cropping out in river beds in the southeast portion of the Hiray flooding flat drainage basin, in contact with linear dunes and in

a layer between the dunes in the Rancho Bueno area and the marine Tepetate Formation. The time when the caliche originated is still unknown but the areal distribution suggests that it is a Pleistocene deposit. The submerging coastal area has a well-developed lagoonal system, which includes tectonic islands of metamorphic and granitic rocks, and sedimentary barrier islands. The coast is formed by rocky and sedimentary cliffs, as well as mangrove swamps and local sandy beaches. Along the coast the spring-tide is mixed, with mean ranges of 1 to 2 m, and average wave heights of 2.5 m (Wright et al., 1973). The continental shelf in the area is broad, with the 200 m isobath about 1 km seaward from the coast. 2.1. Regional setting The climate of Baja California is hot and dry, classified as arid semi-arid by the Ko¨ppen system classification, and rainfall is irregular. The mountainous ridges in the area control the amount of the seasonal rainfall (Wright et al., 1973). Much of the total rainfall results from the rare but strong hurricane storms, which generally occur

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

during the late summer months. They form in the Pacific and proceed across the southern part of the peninsula and into the Gulf of California (Markham, 1972). The average annual potential evapotranspiration is greater than 200 mm, with an annual precipitation of about 50 to 100 mm. The yearly water deficit is nearly 100 mm. The character of the vegetation is semi-desertic (Wright et al., 1973). Dominant northwesterly winds produce a resultant wave force vector toward the southeast, with minor seasonal variations. The long, powerful Pacific swell and short waves dominate the wave regime (Wright et al., 1973). Near the coast, the well-developed local sea breeze system exerts a strong influence on the wave regime (Wright et al., 1973). Land-sea breeze regimes are an important factor in the generation of resultant winds in the area. The La Presa and San Venancio rivers flow intermittently and originate on the western flank of the La Giganta Mountains. The runoff originates at an altitude of about 1,100 m above sea level. Slope direction is to the southwest and crosses through an irregular topography. The streams receive some additional water from tributary streams. Discharge of water for the La Presa drainage basin is through Puerto Chale Estuary within Santa Marina Lagoon, and for San Venancio drainage basin the discharge is to the Pacific coast south of Flor de Malva Spit (Fig. 1). The longitudinal lengths of the drainage basins of La Presa and San Venancio streams are 115 and 93 km respectively and include areas of 1300 and 2400 km respectively. Southeasterly littoral drift, continental shelf sediments, and local intermittent river discharge are the sources for the sediments in the area (Wright et al., 1973, Murillo et al., 1994). Coastal morphology in the mouth of the La Presa intermittent river drainage basin exhibits a well-defined delta front. This river channel seems to be tectonic in origin. The stratigraphies at the sides of the river and estuary are different; the west side shows a basement of deltaic deposits, probably Pleistocene in age (there are no dates available), which are covered by transverse dunes, and on the east side exhibits a basement of the early Holocene linear dunes covered by transverse dunes. 2.2. Late Pleistocene and Holocene climate In the western portion of the United States (Thompson et al., 1993) inferred temperatures at 18 ka BP were below present levels everywhere, with increasingly colder conditions toward the center of the continent. The Southwest was moister than at present. The paleoclimatic record for the West suggests that the region as a whole was wetter at 12 ka than at present. Precipitation remained greater than at present in the Southwest in response to the continuation of stronger-than present onshore flow in winter. When the ice sheet finally disappeared between

143

9 and 6 ka, circulation patterns were a function only of the influence of the enhanced summer insolation. The moister-than-present conditions recorded in the Southwest by the paleoclimatic evidence appear to be largely consistent with an enhancement of summertime ‘‘monsoonal’’ circulation during this interval, resulting from the increased land-sea temperature contrast. As the insolation anomaly gradually diminished to the present, subtle circulation changes accompanied the reduction in the heating of the center of the continent (Thompson et al., 1993).

3. Methods and materials Dune fields were identified by analyzing aerial photographs at a scale 1 : 70,000. Ground-based sampling of the dunes and profiling also defined dune morphology. In accordance with the geomorphological features in the area, thirteen dune field areas were identified: Hiray Norte, Hiray Sur, San Carlos, Carlita, La Herradura, El Cisne, Las Almejas, El Alacra´n, Las Cuevitas, El Datilar, El Rifle Norte, El Rifle Sur, and Rancho Bueno. These areas each include one major type of dune. Fig. 2 shows the boundaries of these dune field areas. In the study area, aeolian deposits that contain mollusk shells were dated by C and amino acid racemization. Marine fossils of the genera Anadara, Chione and Donax were analyzed. Fluvial and aeolian deposits, in which shells were not found, were dated by the thermoluminescence technique. We have given details about dating methods because these are among the first dates for Baja California coastal dune systems. This will enable readers to evaluate these new data. 3.1. Thermoluminescence technique The thermoluminescence (TL) technique has been reviewed and has been applied by different authors to date Quaternary deposits (Dreimanis et al., 1978; Wintle and Huntley, 1982; Mejdahl and Wintle, 1984; Singhvi and Mejdahl, 1985; Aitken, 1985; Singhvi and Wagner, 1986; Berger, 1988; and Mejdahl, 1986). Thermoluminescence in sediments can be thought of as having one light-sensitive component, and one insensitive to light. The insolation before burial reduces partially or completely to zero the thermoluminescence signal of the sediments (the effectiveness of the adjustment to zero depends on the depositional conditions). After burial, the ionization radiation (alpha, beta, gamma) from U, Th, and K displaces light-sensitive electrons (particular kinds of impurities and crystal lattice defects) to a constant rate ((1 Ma dating range). Laboratory measurements can extract a signal of sensitive thermoluminescence light proportional to the burial time (Berger, 1988). Aeolian sediment (light sensitive TL), especially unconsolidated sands,

144

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

Fig. 2. Geomorphic dune field areas, and wind direction (inferred from dune orientation), within the central portion of the Purı´ sima-Iray Magdalena basin.

usually loses a part of its primary TL energy by exposure to sunlight (Wintle, 1982; Wintle and Huntley, 1982). 3.1.1. Sample collection TL subsurface samples were obtained with an auger. The auger was adapted with removable aluminum tubes to recover cores of 7 cm in diameter by 30 cm in length, and a mass of approximately 2 kg. The sampling was done by digging a hole 50 cm square on the surface of the dunes, and, with the auger, a core was retrieved between the interval of 50 to 80 cm depth. The aluminum core with the sample in it, was wrapped immediately after recovery in a black plastic bag, to prevent further exposure to light and preserve the environmental moisture content. At each site where a TL subsurface sample was taken, a TL surface sample was obtained to determine the TL starting point at the time of deposition. The surface sample was collected by sweeping the surface of the dunes to obtain about 2 kg per sample. Then the samples were packed and sent to the TL laboratories. Numeric ages from six distinctive old dune fields, one fluvial area, and three recent coastal dunes, were obtained by the thermoluminescence technique. In total, five surface and fifteen subsurface samples were analyzed. Sixteen dates from fourteen samples were obtained in the

Chemistry Laboratory of the Moscow Lomonosov State University (Table 1), and two dates from two samples were analyzed in the Thermoluminescence Dating Laboratory of the University of Wollongong, Australia (Table 2). The TL samples for the residual correction were obtained at sites 9, 17 and 113. In sites 41 and 105 two TL cores were obtained, which were sent to different laboratories. In site 7 two TL dates were obtained, one from the top and one from the bottom of one core. In site 23 two TL cores were obtained, and a date from each of the cores was obtained. Because two laboratories provided dates, the methods of each will be reviewed for background. 3.1.2. Lomonosov laboratory procedure Uranium-238, Th-232 and K-40 contents were determined from quartered aliquots: 1) By Instrumental Neutron Activation Analysis: Samples, standards and samples for comparison were packed in aluminum foil and aluminum containers and were irradiated in the research nuclear reactor (MIFI) with thermal neutrons (flux&2.8E13 n/cm s) for 15—20 h. The radioactivity was measured (1000— 2000 s) by gamma-spectrometer equipped with

1 1 0.5 0 0.5 0.8 0 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0 0.5 Fluvial-aeolian Fluvial-aeolian Aeolian Aeolian Aeolian Aeolian Aeolian Aeolian Aeolian Aeolian Fluvial-aeolian Aeolian Aeolian Aeolian Aeolian Aeolian 3 12.3 7.1 4.3 4.5 3.8 3.5 5.6 3.2 4.4 0.33($)0.010 0.184($)0.023 0.246($)0.018 0.184($)0.008 0.171($)0.008 0.195($)0.008 0.194($)0.007 0.218($)0.012 0.206($)0.007 0.166($)0.007 95.9($)3.3 28.7($)8.9 45.7($)6.6 32.1($)2.1 19.6($)2.1 30.6($)2.1 25.9($)1.8 37.2($)4.4 40.4($)1.8 17.8($)1.6 10.2($)2.2 8.9($)2.0 24.1($)2.7 8.6($)2.3 12.0($)2.0 9.7($)1.8 14.1($)1.4 12.2($)2.0 11.7($)1.7 13.3($)2.4 12.1 8.02 24.8 9.41 10.5 9.81 13.3 13.2 13.2 13.4

127 41.4 135 54 29.6 69.7 47.1 69 65.2 36.2

0.755 0.693 0.339 0.595 0.662 0.439 0.55 0.539 0.62 0.492

215($)8 329($)13 275($)11 288($)12 343($)14 352($)14 363($)15 359($)14 266($)10 336($)13

7.5 6.5 5.7 3.3 3.4 0.513 0.714 0.824 0.803 0.87 13.4($)3.52 5.5($)1.6 53.3($)4.7 48.8($)2.0 33.9($)1.5 26.12 7.7 64.7 60.8 39 13.8($)1.6 13.0($)2.4 11.6($)2.8 11.9($)2.1 12.3($)2.0

1a 1b 2 3 4 4b 5 6 7 8a 9 10 11 12 13 14a 23 23 3 9 7 7 17 18 37 41 42 90 109 114 113 105

73.5($)18.6 45.7($)12.0 10.5($)1.6 1($)0.1 4.7($)0.7 13($)2.0 0.6($)0.04 11($)2.0 7.1($)0.8 8.8($)0.9 2.5($)0.2 6.3($)2.0 9.7($)1.4 4.4($)0.7 1.4($)0.2 8.7($)1.5

111.80($)30.2 55.90($)14.7 9.50($)1.5 0.90($)0.1 5.20($)0.7 9.30($)1.4 0.50($)0.03 10.40($)1.9 6.10($)0.7 7.10($)0.8 2.50($)0.2 5.80($)1.8 11.50($)1.6 3.90($)0.6 1.10($)0.1 8.50($)1.4

72.5$18.6 44.5$12 9.5$1.64 0$0.41 3.7$0.81 12$2.04 0$0.4 10$2.04 6.1$0.89 7.8$0.98 1.5$0.45 5.3$2.04 8.7$1.45 3.4$0.81 0.4$0.45 7.7$1.55

13.8 16.2 11.1 9.24 10.8

Gammaspectr. by Ac-228 TL no. Site no.

No linear approximation

Linear approximation

Zero mom. corr.

INNA, ppm

Gammaspectr. by Ac-228

INNA, ppm

Ra-U

203($)8 199($)8 243($)10 237($)9 264($)10

0.131($)0.010 0.109($)0.007 0.233($)0.013 0.221($)0.007 0.191($)0.007

Hiray Norte Hiray Norte Carlita San Carlos San Carlos San Carlos La Herradura Carlita El Alacra´n Las Almejas Hiray Sur El Rifle Sur El Rifle Sur Rancho Bueno Rancho Bueno El Rifle Norte

Sample depth Type of material Gammaspectr. by Bi-214] % Area location INNA, ppm Gammaspectr. by Ac-228

P. rad/year Potassium-40 Radium-226 U-238, Thorium-232

Specific activities, Bq/kg

Table 1 Thermoluminescence dates from Lomonosov University Laboratory, of aeolian and aeolian-fluvial deposits within the central portion of the Purı´ sima-Iray Magdalena basin

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

145

Table 2 Thermoluminescence dates from Wollongong University laboratory, of aeolian deposits within the central portion of the Purı´ sima-Iray Magdalena basin Site no.

TL no. Lab no. Plateau region Analysis temp. (°C) Palaeodose (Grays) K content (% by AES) Rb content (ppm assumed) Moisture content (% by weight) Specific activity (Bg/kg U9Th) Cosmic contribution (Gy/yr) Annual radiation dose (Gy/yr) TL age (ka)

41

105

8b W2028 300—500 375 15.8$1.8 1.150$0.005 100$25 1.5$3 74.4$2.3 150$50 2883$59 5.5$0.6

14b W2027 275—500 375 11.6$0.9 1.250$0.005 100$25 3.9$3 57.6$1.8 150$50 2579$54 4.5$0.3

a Germanium ‘‘Ortec’’ detector and 4096-channel analyzers, LP-4900 (Nokia, Finland). Spectrometric standards were used for instrumental calibration (152Eu and Th-232). Interpretation of spectra and elemental composition calculations were carried out according to a PC program (ASPRO and others) (Kolesov et al., 1995). 2) By Gamma-Spectrometric Method with Germanium detector: Contents of Radium-226, a Uranium-238 daughter (by Bi-214), and Thorium-232 (by Ac-228), in samples were obtained with similar gamma-spectrometric equipment and standards. The tabulation of lightsum accumulation rate in quartz was performed by Instrumental Neutron Activation Analysis and by the Gamma-Spectrometric Method with Germanium detector. The errors of tabulation in view of the various contributions to the total palaeo-dose from U, Th and K varied within the limits of 3—7%. Monomineralic fractions of quartz were selected from the 0.10—0.25 mm size fractions by the densitometry method. The calibration of selected quartz on a palaeo-dose was made on a Gamma-400 Irradiator (Cs-137) with an error of palaeo-dose of not more than 6% (variations from 1.5 up to 6%) for at least 10 measurements for each palaeodose. Lightsum of RTL was determined with the Harshaw-4000 TL Analyzer with selection of an individual TL peak under identical conditions for each sample. The value of natural lightsum (So) was determined with a variability of 6—30% for a 95% confidence interval and a number of measurements N"12—16. Variation of dispersion value for So was determined by a set of quartz grains (or impurities) with various palaeo-dose sensitivity and it was a parameter of the samples, but not an error of measurement. Tabulation of RTL-age for the samples younger than approximately 100 ka can be performed in

146

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

the majority of cases for the TL peak with (max.) near 300°C by linear and nonlinear equations (Vlasov and Kulikov, 1989). For each TL sample an age from a linear and a nonlinear approximation equation was obtained. The nonlinear approximation equation, which takes into account the fading factor, gives greater values than the linear equation. The ages obtained by the linear equation were used in the data interpretation. Table 1 shows the thermoluminescence dates, natural radionuclides specific activities (Bq/kg) and palaeo-dose accumulation rate in samples (rad/year). 3.1.3. Wollongong laboratory Upon arrival in the laboratory, TL samples normally consist of two parts; the sample to be dated and a modern analog sample for the surface residual correction. Both specimens were carefully sieved to separate the 90—125 lm fraction, chemically cleansed in dilute HCl, etched in 40% w/w HF, and finally subjected to heavy liquid separation. The sample so prepared consists of better than 99% pure quartz grains. The quartz from the sample under investigation is divided into two parts, one of which is heavily bleached under a UV sunlamp. This exposure effectively removes all of the previously acquired TL leaving only what is termed as the ‘‘unbleachable TL’’. Aliquots of both the bleached and the unbleached quartz were placed on a series of aluminum planchettes, and a number of these were incrementally irradiated using a calibrated Sr-90 plaque source. Each planchette, complete with its sample aliquot, is heated to 500°C at a controlled rate and in an oxygen-free atmosphere. The light emitted (TL) is recorded and in this way it is possible to establish a TL growth curve which relates TL output to the absorbed radiation palaeo-dose. With reference to this curve the measured naturally accumulated sample TL may be converted to absorbed radiation units (Palaeodose P). The surface residual TL correction is determined from the modern analog sample by means of a similar procedure and this correction is applied to the palaeodose value. In the absence of a suitable modern sample the laboratoryinduced unbleachable TL level was assumed which has the effect of maximizing the resultant TL age determined. In the case of an older sample this correction may only represent a small proportion of the total age (Price, 1994). 3.2. Amino acid racemization Amino acid racemization (AAR; epimerization in the case of isolevaine) can be used to provide relative ages of fossiliferous deposits, or, if calibrated using other dating methods, can provide numeric-age estimates. This method has been used routinely for dating fossil material that contains amino acids (Wehmiller, 1984), but primarily for Pleistocene samples, beyond the range of radiocarbon dating. Here we apply this method to esti-

mating ages of Holocene samples based on a rate calibration from radiocarbon-dated samples. It provides a quick and inexpensive means of dating the large numbers of sites analyzed in this study. 3.2.1. Sample collection Samples were collected mostly in the scarps of old coastal dunes, and in holes that were dug in the crests of old dunes to retrieve sand samples for the TL analysis. Also shells of living specimens were collected for the dating calibration. Shells from selected sites were analyzed. Anadara shells were collected at sites, 37, 51, 84, 99, 101, 109, and 113; Chione shells were collected at sites 11, 17, 42, 51, 72, 74, and 113; and Donax shells were collected at site 109. The location of samples is shown in Fig. 3. The Anadara and Chione shells have a mass of about 20 g, while Donax have a mass of about 2 g. Shell preservation state was variable, with one Anadara specimen in poor condition (soft, chalky shell) while most of the others were in good condition. The possible origin of the large marine shells within the dunes is by build-up of middens from human occupation. At the present coast, old scarped dunes contain large shells of pelecypods and gastropods. The dates obtained at Las Cuevitas, and El Datilar dune field areas suggest that humans were living at those places probably during the maximum sea level (ca. 5000 yr BP). Donax shells, which were found in inland dunes (linear dunes), are small specimens which probably were transported by strong winds during an arid episode identified at 0.97$0.05 ka BP (radiocarbon age, at site 109). 3.2.2. Laboratory technique AAR analyses were performed at the Geophysical Laboratory of the Carnegie Institution of Washington. Where possible, material from the hinge of Chione shells was analyzed in order to maintain consistency; racemization rates show significant differences among different parts of the shell (Goodfriend et al., 1997). For Anadara, samples were taken from the margin of the shell, outside the pallial line. Shell pieces were thoroughly cleaned of all chalky or discolored areas using a Dremel motorized tool drill (fitted with a tapered point for fine work, or a small carbide wheel), followed by ultrasonification. Shells were then dipped for 10 —15 s in a dilute HCl solution to remove a thin layer of the shell surface. After rinsing in distilled water and drying, the shells were ground in a mortar and pestle, and an aliquot of 20—60 mg was weighed out for amino acid analysis (Goodfriend, 1987a, b). The shell samples were placed in borosilicate glass tubes and dissolved by addition of a total of 3.1 ml of 12 N HCl/mg of shell material. Tubes were closed under nitrogen and heated at 100°C for 20 h to hydrolyze compounds containing amino acids. The solutions were transferred to plastic microfuge tubes and concentrated

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

147

Fig. 3. Dating site locations, thermoluminescence (TL) dates from aeolian material, and C14, Amino-acid racemization (AAR) dates from mollusk shells, within the central portion of the Purı´ sima-Iray Magdalena basin.

HF was added in 25% excess to precipitate calcium chloride as calcium fluoride. After centrifugation, the supernatant was transferred to another microfuge tube and dried under vacuum. Analyses were carried out by HPLC using post-column derivatization with o-phthaldialdehyde. The D-alloisoleucine/L-isoleucine (or A/I) value was determined by measurement of peak area ratios using a Waters Millennium integration system and were calibrated against a standard mixture of Aile and Ile. The precision of individual A/I measurements is $4% of the ratio. Replicate epimerization analyses were obtained for some shells, (especially those used for calibration) and these values were averaged, except where a large spread of values indicated heating of some parts of the shells. In total, 32A/I values were obtained for shells. Table 3 shows the A/I values for each shell of each genus, the calibrated dates and information about the place of collection. The epimerization rates between genus vary. Chione were calibrated with radiocarbon dates. Fossil Chione shells from sites 72 and 113 and modern shell A/I values and radiocarbon ages were used for the epimerization rate calibration. Radiocarbon ages were calibrated (see below) and a 0 age was used for the modern sample. An A/I value of 0.008 was measured

for modern Chione (this represents the epimerization induced by the preparation procedures). Results for sample S3-35A were not used for calibration because its A/I value was unusually high for its radiocarbon age, indicating that it had probably been heated. The calibration curve is shown in Fig. 5. Simple linear regression yields a slope of 31,460 yr/(A/I), which indicates an epimerization rate of 0.01 per 315 yr. To calculate ages from A/I values of Chione, the induced epimerization (0.008) was subtracted from the measured value and the resultant net epimerization was multiplied by this slope: Age (yr BP)"31,460 (A/I!0.008).

(1)

A calibration for epimerization rates in Anadara was not obtained from radiocarbon analysis but rather by comparison of A/I values of Anadara and Chione from the same samples. Regression of Chione A/I values against Anadara A/I values yielded the following relationship: A/I

"1.164 (A/IAnadara)!0.0007. !FGMLC

(2)

This equation indicates that the epimerization rate for Anadara is about 86% (1/1.164) of that for Chione.

148

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

Fig. 4. Thermoluminescence (TL) average dates and error bars from dune samples, C14 and Amino-acid racemization (AAR) dates from mollusk shells, within the central portion of the Purı´ sima-Iray Magdalena basin. And Holocene sea level curve (Lighty et al., 1982; Shackleton, 1987).

Equation 2 was used to convert Anadara A/I values to equivalent Chione A/I values. Equation 1 was then used to calculate the age.

4. Results and discussion

3.3. Radiocarbon analysis

The Amino-acid epimerization, Thermoluminescence (TL), radiocarbon, and dC values and calibrated dates obtained from the different techniques, for shells and sedimentary deposits of the study area, are shown in Tables 1, 2, 3, 4 and 5. Fig. 3 shows the areal distribution of the dates obtained from the different dating techniques. Fig. 4 shows the Thermoluminescence average date and error bar from aeolian material, and radiocarbon and Amino-acid epimerization dates from mollusk shells within the aeolian deposits. The variation of A/I values observed within individual Holocene shells (e.g. Chione at site 72) and among individual shells (e.g. Donax at site 109) is probably related to heating of the shell either by humans or desert fires. At some sites, where samples for TL, AAR, and C were collected, the ages obtained from these analyses differ widely in some cases, as in the example of the dates obtained at site 109.

In the Geophysical Laboratory, the shell material was pretreated before sending it for radiocarbon and stable isotopic analyses. Shells were fully cleaned, mechanically and chemically. Individual shells of Chione from two sites (72 and 113), an Anadara shell from Site 99, and a bulk sample (15 shells) of Donax from Site 109 were analyzed by Beta Analytic by conventional counting (Table 4). Aliquots were analyzed for stable carbon isotope ratios in the laboratory of K.C. Lohmann at the University of Michigan. The radiocarbon ages were corrected for isotopic fractionation by using measured dC values (Table 5). Calibration of the radiocarbon dates was carried out using the CALIB 3.0 program (Stuiver and Reimer, 1993). The *R value used for calibration was that of Stuiver et al. (1986) for the Pacific Coast of Mexico (#185 yr).

4.1. Dates

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

149

Table 3 Amino-acid epimerization dates from the Carnegie Institute Laboratory, in mollusk shells, within the central portion of the Purı´ sima-Iray Magdalena basin

Genera

Site no.

Sample lab. no.

Sample no.

A/I value

Anadara

37 51 84 99 99 99 99 101 109 113 113 113 113

CD-427 CD-400 CD-403 CD-433 CD-445 CD-401 CD-485 CD-402 CD-428 CD-480 CD-484 CD-399 CD-434

7DA S3-17 S8-36B S3-35A S3-35A S3-35A S3-35A S3-38 TL-11DA S2-21b S2-21 S2-21 S2-21

0.232 0.331 0.14 0.25 0.289 0.301 0.32 0.016 0.518 0.0083 0.14 0.146 0.273

11 17 42 51 72 72 72 74 113 113

CD-407 CD-405 CD-430 CD-482 CD-481 CD-432 CD-404 CD-406 CD-429 CD-483

S6-9 S5-22 TL-9DB S3-17 S4-28 S4-28 S4-28 S8-24B S2-21 S2-21

0.005 1.1 0.13 0.143 0.0446 0.13 0.14 0.014 0.148 0.185

Recent

109 109 109 109 109 109 109 109 109 109

CD-447 CD-437 CD-438 CD-446 CD-439 CD-436 CD-431 CD-435 CD-448 CD-449

11DA 11DA 11DA 11DA 11DA 11DB 11DB 11DB 11DB 11DB

0.0417 0.075 0.078 0.124 0.129 0.103 0.13 0.141 0.159 0.173

Chione

Donax

Age in 1000 B.P. 8.220  4.850



0.313 18.690 Recent




El Alacra´n Las Almejas El Datilar El Datilar El Datilar El Datilar El Datilar Las Cuevitas El Rifle Sur Rancho Bueno Rancho Bueno Rancho Bueno Rancho Bueno

Depth (m)

Place of collection

0—0.8 0.2 0.5 0—0.4 0—0.4 0—0.4 0—0.4 0—0.4 0—0.6 !0.5 1 1 1

Old dune (TL) Old dune Scarp of old dune Scarp of old dune Scarp of old dune Scarp of old dune Scarp of old dune Scarp of old dune Old dune (TL) Lagoon bottom Scarp of old dune Scarp of old dune Scarp of old dune





San Carlos Carlita Hiray Sur Las Almejas Las Almejas Las Almejas Las Almejas Las Cuevitas Rancho Bueno Rancho Bueno

0 5 0—0.15 0 0.2 0.2 0.2 3 1 1

On the beach Coastal dune Old dune (TL) Old dune Old dune Old dune Old dune Scarp of old dune Scarp of old dune Scarp of old dune

    






El El El El El El El El El El

0—0.6 0—0.6 0—0.6 0—0.6 0—0.6 0.6—0.8 0.6—0.8 0.6—0.8 0.6—0.8 0.6—0.8

Old Old Old Old Old Old Old Old Old Old

3.840 4.250


0.188

Rifle Rifle Rifle Rifle Rifle Rifle Rifle Rifle Rifle Rifle

Sur Sur Sur Sur Sur Sur Sur Sur Sur Sur

dune dune dune dune dune dune dune dune dune dune

(TL) (TL) (TL) (TL) (TL) (TL) (TL) (TL) (TL) (TL)

Dates inferred from $1 range (yr B.P.).
Radiocarbon date available.  Chione date available.  Date not calibrated (probably younger than sample 11DB). TL"Hole for thermoluminescence sampling.

At site 109, the upper 60 cm seem to contain shells of mixed ages, e.g. the Donax genus from this depth interval shows variation in the A/I values. Calibrated ages for these shells are not available, however, ages younger than 0.95 ka BP are inferred from the radiocarbon date obtained in a lower interval at the same site (radiocarbon age of 0.97$0.5 ka BP). The shell that was dated for radiocarbon contains higher A/I values than the A/I values obtained in shells within the upper interval. The lower interval from 60 to 80 cm contains only shells from the genus Donax. A radiocarbon date and TL date obtained at this site differ. The explanation of this is that shells in this depth interval are all in the upper 60 cm of the dune, a layer which has probably been modified by

dune reactivation, storms or human influence. The apparent age conflict can probably be explained by two known processes: the epimerization values of the shells are mostly anomalous because of heating, and the TL signal does not seem to have been reset during reactivation of the sands. Heating of the shells is definitely indicated by (1) the large spread in A/I values among individual Donax shells, and (2) the anomalously high A/I value measured in the Anadara shell. In fact, if one takes the lowest A/I value measured in the indiviual Donax shell (0.042) and calibrates that as if it were a Chione shell, one obtains an age that is identical with the radiocarbon date obtained. We can probably be confident that this age is correct for the emplacement of

150

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

the sands in their present position. The TL date, however, is in conflict with this. The TL must represent an older episode of reactivation, or it might represent the time of the dune emplacement. The TL date differences observed at sites 23, 41, and 105 must be related to the different TL core intervals that were analyzed in both laboratories (from 0.5 to 0.8 m depth interval), except for the TL core from site 23, which was not collected exactly at the same depth. Younger TL dates at these sites must represent times of reactivation and/or reworking or redeposition, and the older ages may represent either an older reactivation time or the actual time of the dune emplacement.

Fig. 5. Baja dune Chione racemization rate calibration curve.

4.2. Dune chronology 4.2.1. Late Pleistocene Deposition of alluvial material occurred during oxygen stage 4 (TL 73.5$18.6 ka BP, at site 23), and oxygen stage 3 (TL 45.7$12 ka BP, at site 23). Sea level was at about !50 and !75 m below present, respectively (Shackleton, 1987), and the low relief areas from the Purı´ sima-Iray-Magdalena basin were filled with alluvial deposits. These deposits crop out in the Hiray Norte dune field area, within the Hiray drainage basin (Fig. 2). The TL dates were obtained in the same hole and same interval (at site 23), the differences in ages suggests that one of the cores reached an older deposit. The old alluvial deposits from the Hiray dune field area seem to be, in part, the basement of the Holocene dunes from the Carlita and Las Almejas dune field areas, which are located to the west of the Hiray drainage basin. From the stratigraphic distribution of deltaic deposits from La Presa intermittent river, it can be inferred that the deltaic system was active during Late Pleistocene time. In Recent time, the streams La Presa and San Venancio are the most important contributors of intermittent rain waters and sediment to the lagoonal system and continental shelf. The older dune deposit found in the area is located in La Herradura dune field area (at site 17). The dated shell was collected from the scarp of an old coastal dune at a depth of 1 m (from top to bottom). The dune is about 10 m in height, and is being eroded; blocks have collapsed on the beach. The high A/I value (1.1) indicates a probable Pleistocene age. Aerial photos show that this dune

Table 4 dC13 values from University of Michigan, stable isotope laboratory, in mollusk shells, within central portion of the Purı´ sima-Iray Magdalena basin Site no.

Sample no.

Lab. sample no.

Line/no.

13C (KIS)

18O (KIS)

13C (PDB)

18O (PDB)

Max P

109 113 99 72

11Db-Do S2-21-Ch S3-35A-An S4-28-Ch

2-G23 4-G23 1-G23 3-G23

B/9 A/10 B/10 A/11

3.09$0.04 4.74$0.01 4.44$0.03 4.64$0.02

4.15$0.1 4.12$0.08 3.46$0.08 3.55$0.04

(#)0.91 (#)0.81 (!)0.67 (#)0.71

!0.37 !0.55 !1.09 !1.15

943 1179 797 1359

Table 5 C14 dates from Beta Analytic Inc. Laboratory and Carnegie Institute Laboratory, in mollusk shells, within central portion of the Purı´ sima-Iray Magdalena basin

Site no.

Sample no.

Lab. sample no.

Measured C14 age

13 C/C12 ratio*

Conventional C14

C14 (corr.) (yr B.P.)

C14 (calibr.) (yr B.P.)


$10 range (yr B.P.)


C14 dates

109 113 99 72

11Db-Do S2-21-Ch S3-35A-An S4-28-Ch

92594 92595 92596 92597

1200$40 4470$60 4780$70 550$50

(#)0.91 (#)0.81 (!)0.67 (#)0.71

1620$50 4880$60 5190$70 960$50

1630$50 4900$60 5180$70 970$50

960 4900 5310 430

1030—920 5000—4840 5430—5260 470—360

975$55 4920$80 5345$85 415$55

Corrected for isotopic fractionation (Carnegie Laboratory).
Calibrated using the CALIB 3.0 program with *R"185 $ ( 15) yr.

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

set, oriented northwest-southeast, extends along the coast for about 7 km. This dune ridge is part of the geomorphic features that suggest a Pleistocene coastline. 4.2.2. End of Late Pleistocene-Early Holocene During this time the climate followed a trend toward drier conditions, which led to hot and dry conditions (Elias and Van Devender, 1990). At this time sea level was rising. Evidence for aridity in the area, is observed in dates obtained at 0.5 m depth, in the crest of linear dunes, in the El Rifle Sur (TL 8.7$1.5 ka BP), El Rifle Norte and Hiray dune field areas (linear dunes are 10 m in height), and mega-barchans TL 10.4$1.9 ka BP, at site 18; 7.1$0.8 ka BP, at site 37; (dunes are 10 to 30 m in height) in the Carlita dune field area. In Hiray Sur dune field area linear dunes are present. These dunes were not dated; however, an approximate date of 8.7$1.5 ka BP has been inferred from dates obtained in linear dunes from El Rifle Sur dune field area (at site 109). Probably the inland linear dunes observed in the area were emplaced at the same time. These are 5 to 10 m in height. During the emplacement or reactivation of these dunes the sea level at 14 ka BP was at about !106 m below sea level and rose to !25 m below sea level at around 8 ka BP (Fairbanks, 1989). Probably the Younger Dryas episode, a cold episode between 11 to 10.7 ka BP, which was characterized by an increase of wind intensity and storminess (Fairbanks, 1989), enhanced the aeolian deposition. A brief increase in wind intensity between ca. 11.1 and 10.7 ka BP is recorded by a sharp increase in sediment particle size in aeolian sections along the Nenana River in central Alaska (Bigelow et al., 1990). The formation of linear dunes in the area suggests that the dominant resultant winds were strong, constant, unidirectional southeasterly. The flat relief in this area may also be one of the causes for linear dune emplacement. The dune basement was either a flat eroded surface of marine sedimentary rocks or a thin, hard crust of caliche, which is present at the northeastern boundary of the dune field. 4.2.3. Mid-Holocene During the mid Holocene a ridge of mega-barchan dunes (oriented northwest-southeast) was formed or reactivated in the Las Almejas dune field area (TL 7.8$1.0 ka BP; TL 5.5$0.6 ka BP, W2028, at site 41). It has a length of 40 km, and contains dunes 40 to 60 m in height. The deposition of these dunes may be associated with the approach of the shoreline to its present position (eustatic sea-level rise). At this time the continental shelf was broad and sediment supply abundant. In the El Alacra´n dune field area, dune ridges 30 to 40 m in height oriented west-east were developed (TL 6.1$0.9 ka BP, AAR 8.2 ka BP, at site 37). Younger ages may represent reactivation and/or reworking and redeposition. In this dune field, at the present coast, large mangrove swamps

151

and sabkha deposits are present. A possible climatic influence at this time would be the enhanced monsoonal circulation which relates to greater seasonality of insolation at this latitude. The directions of the dominant winds, as indicated by the dunes, seems to match up with that expected from monsoonal winds (from the SW). Lankford (1977) summarized the work of several authors concerning the Quaternary sea level fluctuations and shoreline formation. He described that when the Holocene transgression slowed at about 5000 yr BP and sea level was at !3 to !4 m, barrier-building processes began to enclose narrow portions of the inner shelf and flooded depressions. Terrigenous as well as marine sedimentation slowly began to prograde the shoreline, thus initiating the Holocene regression. In the study area the Magdalena Lagoonal Complex could be related to this world-wide event. At this time the area barriers were formed, the structural coastal Mesozoic metamorphic mountains became islands. Tombolos, sandy barrier islands and spits enclosed water bodies, which formed both relatively deep and shallow lagoons, and the continental shelf was narrowing and thus producing less sediment. The sediment available was supplying the forming sandy barriers, foredunes, and lower relief inland dunes (size relative to the large dunes located at the back of the present coastline). At the end of mid Holocene through the middle of late Holocene dune emplacement occurred in San Carlos (TL 3.7$0.8 ka BP), Las Almejas (AAR 4.3 ka BP, at site 51; radiocarbon age of 0.97$0.05 ka BP, at the surface of site 72), El Datilar (radiocarbon 5.3$0.1, at site 99, AAR 4.8 ka BP, at site 84), El Rifle Sur (linear dunes, TL 5.3$2.0 ka BP, at site 90), and Rancho Bueno (TL 3.4$0.8 ka BP, at site 114; radiocarbon age of 4.92$ 0.08 ka BP, at site 113) dune field areas. Old dunes in Las Cuevitas dune field area may also have been deposited at this time (no dates are available). The characteristics of these dunes are as follows: In San Carlos dune field area two TL dates, at site 7, were obtained one at 0.5 m depth (TL 3.7$0.8 ka BP) and one at 0.8 m depth (TL 13$2 ka BP). These dunes are 10 to 15 m in height. Some of the troughs are invaded by flooding flats, and some of the flats are sabkhas with saline crusts and pit deposits. The younger age in this area may represent the time of dune reactivation and/or reworking and redeposition. In ¸as Almejas dune field area, the AAR sample at site 51 was obtained in the lower relief dune ridges which are at the base of the high relief mid Holocene dune ridge. The radiocarbon date obtained from the shell that was collected at the surface of this dune field (site 72 suggests that it was part of a midden, this is also suggested from the variation of A/I values in an individual shell. In El Datilar dune field area, coastal dunes of about 5 m in height, flooding flats, and eroded old linear dunes, which are part of the linear dunes of the northeastern boundary. In ¸as Cuevitas

152

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

dune field, dates are not available, however an approximate younger date than 4.25 ka BP could be inferred from dates obtained within the dune field at the northeastern boundary (Las Almejas, site 51), whose stratigraphic position must be older. In this dune field the morphology is patchy, 60% of the area is covered by old dunes 5 to 10 m in height, and 40% of the area is flooding flats. In the Rancho Bueno dune field area the morphology of these dunes, from a reconnaissance aerial view and aerial photographs looks like an irregular sand sheet, but on the ground dunes 20 m in height can be identified. The northern boundary is a sharp contact with the early Holocene linear dunes. 4.2.4. Late Holocene Aridity during Late Holocene has been identified in Hiray Sur dune field area. In this area linear dunes (inferred age of 8.7$1.5 ka BP) and barchans (TL 1.5$0.5 ka BP, at site 42) are present. The barchans are 5 to 10 m in height. These dunes probably were reactivated at this time, as is suggested from the areal distribution in a stratigraphic section, and the presence of the linear dunes in this dune field area. In El Rifle Sur dune field the radiocarbon age of 0.97$0.55 ka BP at site 109 (0.6 to 0.8 m depth interval), must represent either a midden, or storm deposit present at this time. This is also suggested because of the differences in dates obtained at this site (TL 8.7$1.5 ka BP versus radiocarbon age of 0.97$0.55 ka BP). In the area younger coastal dunes, 2 to 5 m in height, are emplaced over middle Holocene dunes, in El Datilar and Las Cuevitas, with an AAR age of 0.31 ka BP at site 101, and at Las Cuevitas an AAR age of 0.19 ka BP (at site 74). Within these younger deposits, pieces of carbon, wood and metal from middens are found. At the present time, in the El Cisne dune field area low relief (lagoonal coast) foredunes, sabkhas, and crescentic dunes 2 to 10 m in height are developing, and also at the top of the sandy barrier islands and spits of the Lagoonal Complex. These large dune extensions are developing in warm-humid weather, with warm winter-hot summers, high-strength sea breezes, strong winter winds, and moderate to high semi-arid vegetation. 4.2.5. Discussion and summary The dune emplacement in the area occurred in colddry as well as warm-humid weather, during rising sea level (large dune fields) and after sea level stabilization. The direction, constancy and intensity of winds, and sand availability seem to be the major controlling factors in the formation of large dunes in the area, factors that were mostly controlled by temperature changes and related to the position of the old coastlines. At low sea level the inner continental shelf was exposed, and a larger area of the sedimentary basin was exposed to erosion. This sediment was deposited on

the low sea level inner continental shelf, reworked into beaches, and incorporated into the active aeolian systems. From the end of the Pleistocene to the Recent, two major events of dune emplacement occurred in the area. The first event began at the end of the Pleistocene, and slowed in the early Holocene, followed by a second event which lasted during the whole middle Holocene. Minor events have been identified for the late Holocene. The climatic differences between these events seem to be mostly related to the intensity and constancy of winds, which is suggested from the old dune orientation. The large coastal and mainland dune fields, have ages between 12$2 ka BP to 4.4$0.7. Pleistocene dates obtained from a fluvial-aeolian deposit, and a shell within La Herradura dune ridge field (high A/I value (Anadara"0.5, at site 109, and Chione "1.1, at site 17), suggest that probably the large dune morphology in the area was emplaced during the Pleistocene, and has been modified by dune reactivation, erosion and aeolian deposition, which gives Holocene dates in the upper parts of the dune surfaces that were sampled for dating. However, the high A/I values may represent heated shells from midden deposits. Stabilization of the Pleistocene shoreline occurred during the Sangamon Interglacial about 80,000 yr BP (Lankford, 1977) between 5 and 8 m above present sea level (Thom, 1967). Stabilization led to the construction of a raised topography of deltaic-lagoon-beach deposits (Price, 1933). Remnants of this ridge-like system are preserved on the modern Mexican coastal plain margins and are commonly associated with present coastal lagoons (Phleger and Lankford, 1974). 4.3. Holocene wind direction From the orientation of dune fields, the wind direction has been inferred (Fig. 2). The morphology of early Holocene deposits suggests that the resultant winds were strong, unidirectional dominant northwesterly winds, and probably westerlies. Evidence for this is the presence of linear dunes within the Hiray Sur, El Rifle Norte, and El Rifle Sur dune field areas, which are oriented northwest-southeast, varying in direction at the southern end of the dune field (west-northwest to east-southeast), and the presence of mega barchans oriented northwest-southeast. Mid-Holocene deposits suggest that the resultant wind was northwesterly (but less dominant than during early Holocene) and westerlies, as evidenced by the presence of dunes oriented northwest-southeast in the San Carlos dune field area, and dunes oriented west-east in the El Rancho Bueno dune field area. Late Holocene and recent deposits suggest the same pattern as during early and mid Holocene, based on the presence of crescentic dunes in the El Cisne area (lagoonal system), oriented west-east (westerly winds), and in Magdalena and

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

Margarita islands (open sea), oriented northwest-southeast (northwesterly winds). Northwesterly winds have been identified in coastal and inland dune field area. The changes of intensity of the Holocene wind must be related to the culmination of the last glacial maximum after which climate shifted to warmer conditions. 4.4. Dune reactivation Young dates were obtained in old dune fields, which suggests that the dunes have been reactivated and/or reworked and redeposited, probably by major storms. In the southern area of the Baja California peninsula, hurricanes are common, originating in the middle latitudes and pass close by, or reach the tip of the peninsula and then continue to the mainland where they disperse. Some of these deviate and pass through the study area. At present, in a warm-humid climate, the inland old dune fields are covered by semi-arid vegetation. At the times of dune reactivation the weather must have been less humid than at present. Two important factors leading to drier conditions may influence the dune reactivation: one is the loss of vegetation cover, and secondly the more effective sediment transport of drier grains. 4.5. Maximum sea level The sharp contact between Las Almejas and Las Cuevitas dune field area; the patchy morphology in Las Cuevitas dune field area (dunes-flooding flats); the presence of sabkhas within dune ridge troughs of the San Carlos and El Alacra´n dune field area, inland dune ridges (40 km long, up to 80 m in height) in Las Almejas dune field area; and the consistent change of morphology at about same height (exact height unknown) from Santo Domingo (northern area) through El Datilar (south area) dune field areas suggest the presence of an old coastline, which may be associated in part to the maximum Holocene sea level, and or the maximum Pleistocene sea level at Oxygen Isotope Sub-stage 5e. The elevation at these areas needs to be accurately determined.

5. Conclusions Holocene dune emplacement appears to have occurred under differing climatic conditions. Wind intensity, resultant wind direction and the duration of the winds seem to play a major role in the development of the dune fields. The annual dominant winds are from the northwest which seems to be dominant throughout most of the Holocene. Humidity and dryness seem not to be major factors in the triggering of dune emplacement in the area, however, it seems that the large inland dune emplacement occurred during dry periods.

153

One factor that has been observed is the relationship of low sea level, and rising sea level, to dune emplacement, which represents cold to warm periods, with strong winds and a broad continental shelf, which supplies a plentiful amount of sediment to the dunes. Aridity at the surface of the Purı´ sima-Iray Magdalena basin has been identified at different times. During early Holocene mega-barchans and linear dunes were formed in the Carlita, El Rifle Sur and El Rifle North dune field areas. Mid-Holocene was characterized by a warm-dry climate. During part of the mid-Holocene dune fields were still active and older dune fields were reactivated and/or reworked or redeposited in Las Almejas and El Alacra´n dune field areas. At about 5 to 6 ka BP the stabilization of sea level developed the Magdalena Lagoonal system. Sandy barrier islands capped with dunes were developed, which slowed the growth of large dune fields on the mainland. At this time in the coastal area dunes were eroded, and in part inundated, forming large flooding flats and sabkha deposits in the old dune troughs. At the stabilization of sea level, the lagoonal system was fully developed, foredunes, dune ridges, and a sand sheet were emplaced, in San Carlos, the El Datilar, Las Cuevitas, El Rifle Sur, and Rancho Bueno dune fields areas, and at the top of the sandy barrier islands. The last period of dune emplacement that was identified occurred at 0.33 ka BP and 0.18 ka BP, when coastal dunes were deposited over middle Holocene dunes. The presence of some younger dates in older deposits seems to represent dune reactivation.

Acknowledgements This paper is a contribution to IGCP Project 367, Late Quaternary Coastal Records of Rapid Change: Application to Present and Future Conditions. This work has been supported by the Union Pacific Oil Company, Nacional Science Foundation, CONACYT, and the University of Southern California (USC). Centro Interdisciplinario de Ciencias Marinas (CICIMAR) and Universidad Auto´noma de Baja California Sur (UABCS) provided logistic support. We thank the researchers of CICIMAR and UABCS who participated in the field trips, specially Dr. Enrique H. Nava-Sanchez. Dr. Y.A. Sapohnikov for his collaboration in the communication with the Lomonosov laboratory researchers. Drs. D. Price, O.A. Kulikov, O.V. Kiryukhin are acknowledged for the hard work of TL dating.

References Aitken, M.J., 1985. Thermoluminescence Dating. New York, Academic Press, 351 p.

154

J.M. Murillo De Nava et al. / Quaternary International 56 (1999) 141—154

Berger, G.W., 1988. Dating Quaternary events by Luminescence. In: Easterbrook, D.J. (Ed.), Dating Quaternary Sediments, Vol. 227, pp. 13—50. Geological Society of America, Special Paper. Bigelow, N., Bege´t, J., Powers, R., 1990. Latest Pleistocene increase in wind intensity recorded in eolian sediments from central Alaska. Quaternary Research 34, 160—168. Chatters, J.C., Hoover, K.A., 1992. Response of the Columbia river fluvial system to Holocene climatic change. Quaternary Research 37, 42—49. Dreimanis, A., Hu¨tt, G., Raukas, A., Whippey, P.W., 1978. Dating methods of Pleistocene deposits and their problems: 1. Thermoluminescence dating. Geoscience Canada 5, 55—60. Elias, S.A., Van Devender, R.R., 1990. Fossil insect evidence for late Quaternary climatic change in the Big Bend region, Chihuahuan desert, Texas. Quaternary Research 34, 249—261. Enzel, Y., Ely, L.L., House, P.K., Baker, V.R., 1996. Magnitude and frequency of Holocene palaeofloods in the southwestern United States: A review and discussion of implications. In: Branson, J., Brown, A.G., Gregory, L.J. (Eds), Global Continental Changes: the Context of Palaeohydrology, Vol. 115, pp. 121—137. Geological Society, Special Publication. Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 342, 637—642. Goodfriend, G.A., 1987a. Chronostratigraphic studies of sediments in the Negev Desert, using amino acid epimerization analysis of land snail shells. Quaternary Research 28, 374—392. Goodfriend, G.A., 1987b. Evaluation of amino-acid racemization/ epimerization dating using radiocarbon-dated fossil land snails. Geology 15, 698—700. Goodfriend, G.A., Flessa, K.W., Hare, P.E., 1997. Variation in amino acid epimerization rates and amino acid composition among shell layers in the bivalve Chione from the Gulf of California. Geochimica et Cosmochimica Acta 61, 1487—1493. Kolesov, G.M., Sapozhnikov, D.Yu., 1995. Neutron activation determination of noble metals in samples of terrestrial and cosmic origin using microfire assay concentration. Analyst 120, 1461—1464. Lankford, R.R., 1977. Coastal Lagoons of Me´xico their origin and classification. In: Wiley, M. (Ed.), Estuarine Processes, Vol. 2, 428 p. New York, Academic Press. Lees, B.G., Yanchou, L., Head, J., 1990. Reconnaissance Thermoluminescence Dating of Northern Australian Coastal Dune Systems. Quaternary Research 34, 169—185. Markham, C.G., 1972. Baja California’s climate. Watherwise 25, 64—76. Mejdahl, V., 1986. Thermoluminescence dating of partially bleached sediments: Nuclear Tracks 10, 711—715. Mejdahl, V., Wintle, A.G., 1984. Thermoluminescence applied to age determination in archaeology and geology. In: Horowitz, Y.S. (Ed.), Thermoluminescence and thermoluminescent dosimetry, Vol. III, pp. 133—190. CRC Press, Boca Raton. Muhs, D.R., Bush, C.A., Stewart, K.C., Rowland, T.R., Crittenden, C.R., 1990. Geochemical evidence of Saharan dust parent material for soils developed on quaternary limestones of Caribbean and Western Atlantic islands. Quaternary Research 33, 157—177.

Murillo, J.M., Osborne, R.H., Gorsline, D.S., 1994. Sources of Beach sand at Creciente Island, Baja California Sur, Me´xico: Fourier grain-shape analysis. Ciencias Marinas 20, 243—266. Petit-Maire, N., 1995. Past Global climatic changes and the tropical arid/semi-arid belt in the north of Africa. In: Flink, C.W., Jr. (Ed.), Holocene Cycles: Climate, Sea Levels, and Sedimentation. Journal of Coastal Research, Special Issue 17, 87—92. Phleger, G.B., Lankford, R.R., 1974. Sedimentos y foraminiferos de la Laguna de Alvarado, Veracruz. Abs. 5°Congreso Nacional de Oceanografı´ a, Guaymas Sonora, 22—25 Oct. 1974. Price, D.M., 1994. TL signatures of quartz grains of different origin. Radiation Measurements 23, 413—417. Price, W.A., 1933. Role of diastrophism in topography of Corpus Christi area, south Texas. Bulletin American Association of Petroleum Geologist 17, 907—962. Raisz, E., 1964. Landforms of Mexico. Prepared for the Geographic Branch of the office Naval Research, scale Map 1 : 4,000.000. Salvador, P.G., Bravard, J.P., 1995. Holocene archaeological occupation cycles in southern France. Journal of Coastal Research, Special issue No. 17: Holocene cycles: Climate, sea levels, and sedimentation 93—94. Shackleton, N.J., 1987. Oxygen isotopes, ice volume and sea level. Quaternary Science Reviews 6, 183—190. Singhvi, A.K., Mejdahl, V., 1985. Thermoluminescence dating of sediments: Nuclear Tracks 10, 137—161. Singhvi, A.K., Wagner, G.A., 1986. Thermoluminescence dating and its applications to young sedimentary deposits. In: Hurford, A.J., Ja¨ger, E., Tencate, I.A.M. (Eds), Dating young sediments: Bangkok, Thailand, UNCCOP Technical Secretariat, pp. 159—197. Stuiver, M., Reimer, P.J., 1993. Extended C data base and revised CALIB 3.0 C age calibration program. Radiocarbon 35, 215—230. Stuiver, M., Pearson, G.W., Braziunas, T., 1986. Radiocarbon age calibration of marine samples back to 9000 CAL, UR BP. Radiocarbon 28, 980—1021. Thom, B.G., 1967. Mangrove ecology and deltaic geomorphology: Tabasco, Mexico. Journal of Ecology 55, 301—343. Thompson, R.S., Whitlock, C., Bartlein, P.J., Harrison, S.P., Spaulding, W.G., 1993. Climatic Changes in the Western United States since 18,000 yr BP. In: Wright, H.E. Jr., Kutzbach, J.E., Webb III, T., Ruddiman, W.F., Street-Perrott, F.A., Bartlein, P.J. (Eds), Global Climates since the Last Glacial Maximum, pp. 468—513. Univerity of Minnesota Press. Vlasov, V.K., Kulikov, O.A., 1989. Radiothermoluminescence Dating and Applications to Pleistocene Sediments. Physics and Chemistry of Minerals. Springer-Verlag, 16, pp. 551—558. Wehmiller, J.F., 1984. Relative and absolute dating of Quaternary mollusks with amino-acid racemization: evaluation, applications and questions. In: Mahaney, W.C. (Ed.), Quaternary Dating Methods, pp. 171—193. Elsevier, Amsterdam. Wintle, A.G., 1982. Thermoluminescence properties of fine-grain minerals in loess. Soil Sciences 134, 164—170. Wintle, A.G., Huntly, D.J., 1982. Thermoluminescence dating of sediments. Quaternary Science Reviews 1, 31—53. Wright, L.D., Roberts, H.H., Coleman, J.M., Kupfer, R.L., Bowden, L.W., 1973. Process-form variability of multiclass coasts: Baja California. Technical Report No. 137. Coastal Studies Institute Louisiana State University, Baton Rouge, Lousiana 70803, 54 p.