A provisional aminostratigraphical framework for late Quaternary marine deposits in Buenos Aires province, Argentina

MARINE Marine Geology 128 (1995) 85-104

A provisional aminostratigraphical framework for late Quaternary marine deposits in Buenos Aires province, Argentina M.L. Aguirre a, D.Q. Bowen b, G.A. Sykes b, R.C. Whatley ’ ’ Division Paleozoologia Invertebrados, Muse0 de Ciencias Naturales, Paseo de1 Bosque S/N, 1900 La Plats, Argentina b Department of Earth Sciences, University of Wales, Card@ CF13 YE, UK ’ Institute of Earth Studies, University College of Wales, Aberystwyth, Dyfed SY23 3DB,

UK

Received 11 January 1994; revision accepted 30 March 1995

Abstract Amino acid analysis was carried out on 5 bivalve taxa (Mactra isabelleana, Corbula patagonica, Tagelus plebeius, Macoma uruguayensis and Noetia bisulcata). Data from the two most common (Mactra isabelleana and Corbula patagonica) are reported in detail. Samples were collected from Holocene shorelines (ca. 2000-6890 14C yr B.P) along ca. 350 km of the Buenos Aires Province, Argentina. The aminostratigraphy is in general agreement with the palaeoecological evidence and radiocarbon ages. The deposits of the area of Punta Indi*Punta Piedras in the north, which show the highest D/L ratios, have a different geometry and a molluscan fauna of warmer water affinity than the remaining beach ridges, and must be older. They could correspond with the climatic optimum (‘Hypsithennal’). The deposits of Samborombon and Mar Chiquita probably started accumulating immediately after deposition of the Punta Indio-Punta Piedras deposits. Deposition at Mar Chiquita appears to have continued longer than at Samborombon. Older D/L ratios from pre-Holocene marine deposits are difficult to interpret without geochronological calibration.

1. Introduction Aminostratigraphy is based on the time dependent racemization or epimerization of amino acids in fossils. Amino acids in the protein of living organisms are all in the L-configuration, but after death L-amino acids interconvert (racemize or epimerize) with D-amino acids until an equilibrium racemic mixture is reached. The greater the ratio of D-amino acid/L-amino acid measured in a fossil, the older it is. In this research the epimerization of L-isoleucine (L-Ile) to D-alloisoleucine (D-aile) is used as the basis of aminostratigraphical correlation. Interpretation of D/L ratios is sometimes complicated because environmental factors, such as temperature, and species can affect the rate of epimerization. In this study, however, the 0025-3227/95/$9.50 0 1995 Elsevier Science B.V. All rights reserved SSDZ 0025-3227(95)00058-5

samples were collected from a limited area (350 km north to south) in which the integrated palaeotemperature history was probably similar. Calibration of the ratios with radiocarbon ages allows the effect of temperature fluctuations during the Holocene within the study area to be minimised. But different exposure history (i.e., direct sun) of the shells, and the temperature for individual samples, is to be expected within such nearshore environments. Aminostratigraphy has been extensively used to correlate marine events along the northern Atlantic and Pacific coasts with the oxygen isotope stratigraphy (Bowen and Sykes, 1988; Miller and Mangerud, 1985; Bowen et al., 1985; Wehmiller et al., 1978). Recent reviews include those by Sykes ( 1991) and Wehmiller ( 1993). Examples of amino-

86

M.L. Aquirre et al.!Marine

stratigraphical studies in the southern hemisphere (SH) include: Argentina, Rutter et al. (1989, 1990); the South Pacific, Hearty and Miller (1987); Peru and Chile, Hsu et al. ( 1989), Wehmiller (1990), Leonard et al. (1987), Leonard and Wehmiller (1992); Australia, Cann et al. (1988), Hearty and Aharon (1988), Thorn and MurrayWallace (1988), Nichol and Murray-Wallace (1992), Murray-Wallace and Kimber (1987), and Murray-Wallace and Belperio ( 199 1). Only a few of these investigated late Quaternary marine deposits: that is, younger than oxygen substage 5e (Murray-Wallace et al., 1993; Rutter et al., 1990). The data are from different molluscan taxa, thus making correlation difficult. The coastal plain of northeastern Buenos Aires Province in Argentina, between Magdalena and Mar Chiquita (Fig. 1) contains abundant shelly marine deposits that correspond mainly with raised beach ridges at different elevations. These may correspond with three marine transgressions, two during the late Pleistocene (Gonzalez et al., 1986, 1988a,b), the other, with the most extensive deposits, during the mid-Holocene. A data bank of radiocarbon ages exists for molluscan shells from these deposits (Table 1). The aim of this paper is to present the amino acid data from the marine Holocene deposits along the northeastern Bonaerensian littoral area, and to propose a geochronology for them. The radiocarbon ages are sometimes of limited use because they only indicate a ‘probable geological age’ (Gonzalez et al., 1983), as many of the deposits are reworked. In principal, amino acid geochronology seems particularly applicable to these sites when individual shell fragments can be dated, the youngest shell fragment gives a maximum age for a deposit (Goodfriend, 1989). Some deposits (late Pleistocene) are beyond the range of radiocarbon dating. Thus amino acid geochronology may help to date these earlier deposits.

Geology 128 i 1995) 85-104

2. Previous work The earliest studies of these deposits were carried out by Darwin ( 1846) and D’Orbigny ( 1842-l 844) in the mid 19th century. More recent stratigraphical and palaeontological studies were by Fidalgo et al. (1973, 1975), Tonni and Fidalgo (1978), Schnack et al. (1982), Spalletti et al. (1987), Weiler et al. (1987), Violante (1988), Camacho (1966), Aguirre (1990, 1993a,b), Tonni et al. (1992), and Codignotto and Aguirre (1993). The number and extent of Quaternary marine transgressions which affected the Bonaerensian littoral area have not yet been established. Some recognise two high sea-level stands of Holocene and late-Pleistocene age, but they differ in correlating the latter with the Last Interglacial (sub-stage 5e), and/or a “late Pleistocene interstadial” (Fidalgo and Tonni, 1982). Fidalgo (1979) recognised three marine transgressions in the Samborombon Bay sector (Fig. l), one during the Holocene, another of Late Pleistocene age, and the third at the Pleistocene/Holocene boundary. This is based on three (marine) lithostratigraphical units (Fig. 2): (3) Las Escobas Formation (mid-Holocene transgression, + 2.5 - 5 m above m.s.1.) (2) Destacamento Rio Salado Formation (deposits of a coastal lagoon at the Pleistocene/ Holocene boundary, at or below present sea level) ( 1) Pascua Formation (Late Pleistocene transgression, possibly of Sangamon age at + 3 - 6 m above m.s.1.) In the Magdalena and Mar Chiquita areas, Weiler et al. (1987) and Weiler and Gonzalez (1988) proposed a mid-Holocene transgression, and have also recognised Pleistocene sediments, some of which were deposited during the ‘Last Interglacial’ (Sangamon), while others were thought to have been deposited in an interstadial of the Last Glaciation. In the south, along the

Fig. 1. Study Area. Landforms, localities (I to 11: Holocene; Magdalena and Puente de Pascua: Pleistocene) and samples (A201; WA-7, etc.) studied. S= Sangamon Interglacial? (strongly cemented deposits with interbedded caliche; estuarine sediments), I= MidWisconsinian Interstadial (mollusc shells dated at 32,000+ 1700 yrs. B.P.; estuarine sediments); Holocene: brown-reddish eolian silts without shells. The numerical values (0.210, etc.) correspond to the minimum amino acid ratios on Mama ixzhelleana, except for Magdalena (AC-0911) and lot. 7 (A204, WA-7) where they were obtained on Tugrluspleheius (the complete list of ages obtained are shown in the Appendix). Partially modified from Codignotto and Aguirre (1993) and Gonzalez et al. ( 1986).

87

M. L. Aquirre et al.jMarine Geology 128 (1995) 85-104

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Samples

1

Locality

(7,9)

(5) 6150; 6459; 6764 (3) 4860; 5580; 4960 (6) 6980 (6) max. 5000; 2900 (7,8) ea. 2700-2900

(3) 3050; 4920; 5934; 6056 5630 (10) 4000-4500

(1) ca. 3000 (2) 7890 (?); 3762; 4067

4460-7600

ca. 3000 (2)

r“C yrs B.P.

0.121 0.053 0.066

0.066 0.060 0.065 0.056

0.080 0.050 0.088 0.076 0.048 0.069 0.058 0.078 0.063

0.110

0.070 0.109 0.110

0.135 0.115 0.115 0.123

0.136 0.081 0.110 0.099 0.098 0.085 0.075 0.130 0.094

0.148

Mactra Mm. Max.

1.000 0.080 0.097

0.096 0.076 0.082 0.106

0.098 0.064 0.099 0.086 0.082 0.077 0.063 0.108 0.074

0.128

Mean

0.022 0.022 0.021

0.025 0.016 0.015 0.042

0.016 0.011 0.009 0.012 0.020 0.008 0.007 0.016 0.012

0.012

SD

4 5 4

10 10 10 6

10 5 5 5 5 5 5 10 5

10

N

0.070

0.120 0.099 0.065

0.120

0.131 0.186 0.179 0.114 0.109

0.112

0.146 0.188 0.116

0.187

0.231 0.157 0.172

0.195

Corbula Min. Max.

0.096

0.129 0.126 0.084

0.148

0.155 0.186 0.197 0.126 0.140

Mean

0.018

0.012 0.042 0.023

0.021

0.021 0.025 0.025

0.028

SD

10

4 4 4

10

4 1 5 5 4

N

0.101

0.053

0.123 0.106

0.120

0.106

0.173 1.960

Tagelus Min. Max.

Synthesis of information on samples, radiocarbon dates, and D/L ratios of species in the Holocene localities studied. SD = standard deviation; N = number of shells analysed. Stratigraphic position of samples for each locality follows a corresponding for each locality were published by: (1) Cortelezzi and Lermann (1971); (2) Gomez et al. (1985); (3) Gbmez et al. (1988); (6) Codignotto and Aguirre (1993); (7) Fasano et al. ( 1982); (8) Violante (1988); (9) Schnack

Table 1

0.114

0.075

0.153 0.147

Mean

0.011

0.021

0.026 0.400

SD

3

5

3 7

N

0.123 0.115

0.173 0.184

Noetia Min. Max.

Min. and Max. = minimum and decreasing order of depth. The Fidalgo et al. (1981); (4) Figini et al. ( 1982); and (10) Fidalgo

0.149 0.144

Mean

0.013 0.021

SD

10 10

N

maximum values; radiocarbon dates et al. (1990); (5) (1979).

M. L. Aquirre et aL/Marine

CLIMATIC OSCILLATIONS

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89

Geology 128 (1995) 85-104

: LAS

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DESTACAMENTO RIO SALAD0 FORMATION _--__-_------_ PAMPIANO FM.

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Fig. 2. Stratigraphical pattern of the area of study. M=marine,

coastal area of Mar Chiquita, Schnack et al. (1982) recognised a Holocene transgression, while Isla et al. (1986) described Pleistocene marine deposits at +4 m above m.s.1. near Mar de1 Plata, some 1 km west of the Holocene ridges. Radiocarbon ages are available for some of these deposits (Fidalgo, 1979; Figini et al., 1984; Fasano et al., 1982, 1987; Gdmez et al., 1985, 1988; Gonzalez et al., 1986; Gonzalez and Weiler, 1988a,b), but the reworking of shelly material makes some of them less reliable.

3. Geological setting The coastal plain from Magdalena to Mar Chiquita is typical of the ‘Pampas’. It is crossed by the Samborombon and Salado rivers in the

FL=fluvial

and E=eolian

sediments. Taken from Aguirre (1993a).

north. In the south the Mar Chiquita coastal lagoon dominates the area. In general it is a low coast, the area of San Clemente to Punta Medanos is one of progradation, while the Villa Gessel-Mar de1 Plata region is one of erosion. Presently Samborombon Bay is influenced by the Rio de La Plata, and by storms from the southwest, but the coastline appears relatively stable, although subsidence is suspected at the Salado Depression (Isla et al., 1990; Codignotto and Aguirre, 1993). Along its 350 km length the study area is likely to have experienced the same integrated palaeotemperatures. The contemporary mean annual maximum temperature is 18”, while the mean annual minimum temperature is 13” (Gonzalez et al., 1988). The littoral region between Punta Indio and Punta Rasa consists of tidal flats near the mouth of the Rio de La Plata, while between

90

M.L. Aquirre et d/Marine

Punta Rasa and Mar de1 Plata it corresponds to the South Atlantic zone known as the Argentine Sea. The position of former sea levels along the Argentine coastal area has been mainly determined by tectonic displacements, glacio-isostatic adjustment of the area surrounding Antarctica and isostatic sea-level change (Rutter et al., 1989). Quaternary marine sediments overlie the continental Pampiano Formation of Pliocene to late Pleistocene age (Fig. 2). Most of them are represented by characteristic mid-Holocene littoral beach ridges which make up the Cerro de la Gloria Member of Las Escobas Formation which unconformably overlies Pleistocene deposits. The Holocene deposits are approximately synchronous with other similar marine deposits recognized along the eastern South American coast (Altena, 1971; Suguio et al., 1986; Martin et al., 1986; Turcq et al., 1986; Villwock et al., 1990; Sprechmann, 1978; Farinati, 1985a,b; Gonzalez et al., 1983; Fasano et al., 1987; Feruglio, 1950; Codignotto, 1983; Fasano et al., 1984; Rabassa, 1987a) and worldwide (Newman et al., 1984).

4. Materials and methods 4.1. Sampling Shells from five areas along northeastern Buenos Aires Province (Fig. 1) were collected for amino acid analysis. Most localities correspond to, or are equivalent to littoral beach ridges of the Cerro de la Gloria Member of the Holocene Las Escobas Formation (locations 1-6, 8-9, 10 and 11). These beach ridges belong to the barrier islands and barrier spits (Codignotto and Aguirre (1993). A few samples were collected from a coastal lagoon facies of the same formation (Canal 18 Member) (location 7). Descriptions of the deposits from these localities have been published by Aguirre (1993b) and Codignotto and Aguirre (1993). Samples were collected from precise stratigraphical levels, and from a minimum depth of 30 cm. Only well-preserved shells were selected for analysis. The taxa, however, are representative of the entire fossil assemblages.

Geology 128 (1995) 85-104

For comparative purposes, Pleistocene shells were collected from the Pascua Formation (Puente de Pascua; sample A210, Fig. 1 ), Canada de Arregui, Magdalena, (sample AC-091 1), and from Holocene beds south of the main study area (Bahia Anegada, AC-1014, ca. 39.5’S). 4.2. Sample preparation and analysis Standard sample techniques were used (Miller et al., 1987). Inter-laboratory standards used in this study gave the following D-alloisoleucine/Lisoleucine (D-aile/L-ile) ratios: ILC A Powder 0.164 (0.174+0.033), ILC B Powder 0.480 (0.525kO.055) and ILC C Powder 1.069 ( 1.07 1 + 0.112). The original data ( Wehmiller, 1984) are given in parentheses. First, the samples for analysis were cleaned mechanically under running water with a soft brush. They were further cleaned by dissolution of the outer l/3 of the shells with 2 A4 HCl, followed by ultrasonication for 30 seconds, and 5 washes with 18 Ma water. After drying, the samples were transferred to a clean 5 ml vial and dissolved in 7 A4 HCl containing 12.5 uA4 nor-leucine (0.02 ml/mg shell). After flushing with oxygen free nitrogen, the samples were hydrolysed at 110°C for 22 hours. The hydrolysed samples were dried in a vacuum desiccator over KOH and then rehydrated with low pH solution (18 MS2 water with enough HCl added to drop the pH below 2). D-alloisoleucine/L-isoleucine (D/L) ratios were measured by liquid chromatography using a modified method of Benson and Hare (1975). Chromatography was on a 25 x 0.21 cm column packed with ion exchange resin (5.5 u ion exchange resin supplied by St. John Associates Inc.) Samples were loaded onto the column via either by a Rheodyne 7 125 valve, or a LKB 2135 autoinjector. Amino acids were eluted from the column by a step gradient of 67 mA4 sodium citrate buffer pH 3.25 or 3.8 and the column was cleaned using 33 mM sodium borate buffer containing 179 mM NaCl and 2.7 mM EDTA. Buffer flow was maintained at 0.1 ml/min using an Eldex A-30-S pump. Eluant from the column was mixed with o-phthaldialdehyde (OPA) reagent, 0.49 M potassium borate pH 10.5, containing 0.03% w/v Brij 35 and

M.L. Aquirre et al.IMarine Geology I28 (1995) 85-104

15 ml/l OPA solution (0.750 g OPA, 15 ml methanol, 300 ul mercaptoethanol). OPA reagent was pumped at 0.11 ml/min by a Gilson minipuls 2 pump. After passing through a reaction coil (80 x 0.005 cm) the amino acids were detected as their OPA derivatives by a Gilson 121 fluorometer (using OPA filters). Control of the step gradient and data analysis was carried out using a Nelson Analytical 900 series interface coupled to a PC running Nelson Analytical 2600 software. 4.3. Molluscs

and radiocarbon ages

Direct comparisons between amino acid D/L ratios should be made between the same species (King and Hare, 1972), but it is not always possible to collect a single species over an extensive area ( Wehmiller, 1990; Rutter and Schnack, 1990; Rutter et al., 1989, 1990), as is the case in the study area where different biofacies are characterised by molluscan associations differing in taxonomic composition and abundance (Aguirre, 1990). If, however, the differences in the epimerization rate between taxa are known, it may be possible to address this problem (see Bowen et al., 1985, for a statistical treatment of species differences). Because of the different species used, we are unable at present to compare our results with those in Patagonia (Rutter et al., 1989). Radiocarbon ages on marine shells (bivalves and gastropods) suggest minimum ages for the deposits between ca. 1400 and 7000 14C yr B.P. (Cortelezzi, 1977; Fidalgo et al., 1981; Figini et al., 1984; Gomez et al., 1985, 1988; Fasano et al., 1982, 1987; Gonzalez et al., 1986; Weiler et al., 1988; Violante, 1988; Codignotto and Aguirre, 1993) (Table 1). A range from ca. 2500 to 6890 yr B.P. is characteristic of Samborombon Bay, and between ca. 1400 and 5000 yr B.P. of the Mar Chiquita area. Some of the radiocarbon ages (Fasano et al., 1982; Violante, 1988) were assayed on samples in the ‘life position’ or ‘growing position’ of molluscan shells. It is likely, however, that the marine shelly accumulations studied here are not in situ assemblages, but are aggregates of shells from different communities and generations, which is not uncommon for storm-wave beach deposits

91

along the supralittoral zone or submerged sublittoral bars in the nearshore (Aguirre, 1993b). This is supported by not being able to find many, or no, bivalve shells in living position in the field (Aguirre, 1988). Only in the coastal lagoon facies of Las Escobas Formation (Canal 18 Member) were articulated shells of Tagelus plebeius in living position collected, where they were abundant. Whole and fragmented shells have been subject to the effects of post-mortem transport and other taphonomic processes, resulting in different rates of surficial abrasion, dissolution, breakage, size selection and resultant orientation within the beds. As the result of reworking, radiocarbon ages from most of these localities cannot distinguish between different beach ridge deposits. The range of ages from each beach ridge exceeds the difference in ages between the ridges (Gomez et al., 1988, Codignotto and Aguirre). It is reasonable to assume that the exposure history of different shells will have varied. Reworking implies that some shells might have been exposed to higher temperatures in the shallowest portions of the beach ridge deposits for some time, as compared with others not so re-worked, and more deeply buried samples. Such a potential “thermal effect” suggests that even samples buried below 30 cm, at the time of collection, may have been affected earlier in their history. Thus, an unavoidable result of such mixing, and different thermal effects, is the scatter cf ages obtained both by radiocarbon dating (Table 1) and amino acid analysis (Fig. 3). As the result of reworking, and thermal effects, the lowest D/L ratio will represent a maximum age for a deposit. The study area and the taxa used for dating were selected on the following criteria: (1) individual localities have similar current mean annual temperatures (15”~16”C, Fidalgo, 1979); (2) taxa that are common throughout the study area; (3) good preservation; and (4) species showing life habits and habitats less exposed to transportation and reworking (moderate to deep infaunal elements of soft bottom environments).

M. L. Aquirre et al. JMarine Geology 128 (1995) 85-104

-

0.207

(a) 0.16.

.

8 0 0.12-

; .

0.08-

, 8 . .

8

’ 8 . . ; ; .

: 8

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0.2

0.1

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Z2P

8 G a Sample 222 Locality 11

$33

4 Fig. 3. D/L ratios of Mactra

isabelleana

(a) and Corbula patagonica,

5. Data The radiocarbon ages and D/L ratios are in broad agreement throughout the study area. The

Tagelus plebeius and Noetia bisulcata (b).

innermost ridges gave older ages: that is, localities 1 and 9. At some localities (4, 8 and 11) the amino acid ratios are inverted (Table 1), which could either be the result of taphonomic processes, or of

M. L. Aquirre et al/Marine

a temperature effect. These units were likely to have been extensively reworked, but the lowest D/L ratio gives a maximum age for the entire unit. Radiocarbon ages from barrier islands of Samborombon Bay give similar difficulties: the oldest ages between 4800 and 5580 yr B.P. were given by shells in the centre of the unit, whereas shells at the top of the unit gave ages of between 4600 and 4440 yr B.P., and at the bottom of between 4100 and 4960 yr B.P. (Gomez et al., 1988; Codignotto and Aguirre, 1993). Five bivalve taxa were included in this study (see Appendix). Of these, only Mactra isabelleana and Corbula patagonica are considered in detail. Macoma uraguayensis occurred at 4 sites but amino acid analysis was only carried out on 2 of them (Locality 7: D/L ratio 0.127, 0.068,0.096; Locality 8: D/L ratio 0.077). Similarly D/L ratios from Noetia bisulcata were only measured from two beds at Locality 1 (samples 115 and 201). D/L ratios from Tagelus plebeius were measured at 5 localities, together with one sample from south of the study area. D/L ratios from samples of Mactra isabelleana had the lowest coefficient of variance of all the species in this study, and interpretation of this is hampered because it is the slowest epimerizing species analysed. D/L ratios from Mactra isabelleana, therefore, were less able to distinguish adjacent beach ridges. The ratios of different beds at Localities 3 and 4 were inverted (WA-l, WA-2; WA-4 to WA-6), probably as the result of reworking of beach ridge deposits. D/L ratios from Corbula patagonica, in spite of larger coefficients of variance, were the most successful in distinguishing sea-level events close in time. 5’.I. Pleistocene deposits D/L ratios of shells from two Pleistocene deposits were significantly higher than for the Holocene beach ridges. Magdalena

The reliability of the radiocarbon dates for these deposits, representing minimum ages and being near the limit of the method, has been discussed by Gonzalez et al. (1988b) (see also Isla, 1989). A minimum radiocarbon age of 32,000 is available

Geology 128 (1995) 85-104

93

(M. Gonzalez, sample AC-09 11, INGEIS Laboratory, unpubl. report). The D/L ratio on Tagelusplebeius is 0.556 f 0.07 (18). This is considerably higher than the oldest Holocene ratio on the same species (0.153f0.026, 18). Puente de Pascua

The oldest Holocene ratio on Mactra isabelleana is 0.129+0.012 (lo), whereas the D/L ratio obtained in P. de Pascua is 0.387 k .052 (16). Interpretation of these Pleistocene data is difficult because Tagehs epimerizes faster than Mactra, but it seems likely that both Pleistocene deposits belong to the same high sea-level stand. The higher amino acid ratios for these deposits points to a Pleistocene age, and are likely to be of stage 5 age or older. A correlation with stage 5 is supported by Mitterer and Kriauskal’s (1989) parabolic kinetic model. This gives ages of 106 ka for Magdalena and 123 ka for Puente de Pascua (Fig. 4), however, these ages can only be considered provisional because the model could only be calibrated using Holocene samples. Certainly in view of data from oxygen isotopes and uplifted coral terraces (Shackleton, 1987; Matthews, 1990) it is unlikely that they are “Wisconsinan” in age, as was previously suggested by Gonzalez et al. (1986), and Weiler et al. (1988) for similar marine deposits from 33”s to 40”s along the Argentine coastal area (Entre Rios, Isla Martin Garcia, Magdalena, Mar Chiquita, Colorado River delta), and the similar suggestion by Fasano et al. (1984), Isla (1989) and Rutter et al. (1989, 1990) for late Pleistocene coastal deposits between Mar de1 Plata and Patagonia. Rabassa and Clapperton (1990) placed an interstadial at ca. 60-30 kyr B.P. for southern South America, Isla (1989) provided evidence for the Wisconsinian interstadial ca. 30-40 kyr B.P. in Patagonia and other areas of the world, while Clapperton ( 1993) established that the main glacier advance in South America was in progress soon after 34 kyr B.P. until ca. 27 kyr B.P. Evidence for interstadial conditions during the interval 35-27 kyr B.P., before glacier advances occurred after 27 kyr B.P., have been reported for Alaska, U.S.A., Tasmania and New Zealand, and recent

ML.

Aquirre et ccl.JMarine Geology 128 (1995) X-104

0.60

0

100000

50000

150000

Age calibrated to Radiocarbon Fig. 4. Parabolic kinetic standard deviation.

modelling

of D/L ratios

from

Tag&s

evidence from the Magellan Strait in southernmost Chile and the Antarctic Peninsula (Alexander Island) suggest the possibility of a higher than present sea level (implying warmer conditions than during the subsequent glacier advances) ca. 35 and 32 ka respectively (Clapperton, pers. commun., November 1994, submitted). Very little is known about this “interstadial” in South America and the scarce radiocarbon ages (26-36 kyr B.P.) available are in disagreement with the oxygen isotope record which supports a global low sea level and cold conditions in that age range instead of a relatively warm event as the occurrence of an interstadial would imply. Most of the published glaciological evidence seems to argue against a sea level higher than present at that time. The existence of deposits of a “midWisconsinan” high stand of sea level would necessarily have to be the result of uplift, and they would need to be confirmed by independent and

and Muctru.

200000

Years BP

the boxes show D/L ratios

and age calculated

to one

reliable geochronological age estimates. The sort of uplift required for this event only occurs at plate margins (Bloom and Yonekura, 1990). 5.2. Holocene deposits Subdivision of the Holocene deposits by aminostratigraphy is difficult but not impossible. The amino acid analyses show the occurrence of overlapping stratigraphical units (Fig. 3). The events in question fashioned the landforms described by Codignotto and Aguirre (1993) for the Punta Indio Punta Rasa area, and by Fasano et al. (1982), and Violante (1988), for the surrounding area of Mar Chiquita. Radiocarbon ages for the minor barrier spits (locations 1 and 2) are: 7600 and 4460 yr B.P. (Cortelezzi and Lermann, 1971) and 3100 yr B.P. (Gomez et al., 1985) The D/L ratios are: Corbulu 0.157-JO.036 (19) (the minimum ratio is 0.103, while the maximum one is 0.231); and Mactra

M. L. Aquirre et al./Marine Geology 128 (1995) 85-104

0.113 * 0.020 (20) (minimum 0.080, maximum 0.148). These deposits from Punta Indio and Punta Piedras exhibit higher D/L ratios because they are older, or because they were exposed to higher temperatures because of their lower latitude (Fig. 1). They would, however, have experienced the same temperatures as neighbouring localities in the north of Samborombon Bay, where samples yielded lower ratios. The beach ridges of these minor barrier spits have a different geometry because they are truncated and have been formed by a different system of longshore sand transport than the other ridges. The older age for the Punta Indio Punta Piedras deposits is supported by palaeoecological evidence which suggest correlation with the Hypsithermal (Aguirre, 1993a) between 7000 and 6000 yr B.P. (Rabassa and Clapperton, 1990). Radiocarbon ages for the barrier islands (locations 4, 5, 6, 8, and 9) range from 6980 to 2500 yr B.P. (Codignotto and Aguirre, 1993). The D/L ratios are: Corbula min 0.099 max 0.188 mean 0.139* .026 (18) Mactra min 0.048 max 0.135 mean 0.083k.020 (75) Radiocarbon dating of beach ridges in the barrier islands are difficult to interpret, dates from equivalent positions in the same beach ridge sometimes vary by 2000 years. However, they suggest that the beach ridge may have accumulated from southwest to northeast between ca. 7000 to ca. 2500 yr B.P. Aminostratigraphical evidence for the barrier islands come mainly from D/L ratios on Mactra isabelleana. Corbula patagonica ratios are not clearly younger than for the minor barrier spits but this may be an artefact caused by a lack of specimens. Synthesis of aminostratigraphical, radiocarbon dating, stratigraphy and palaeontology suggest that the barrier islands started to accumulate immediately after deposition of the minor barrier spits, they continued to accumulate over the next ca. 3500 years and were continually reworked during this period. The southern barrier spits (locations 10 and 11) have radiocarbon ages between 5000 and 2700 yr B.P. (Fasano et al., 1982; Violante, 1988; Weiler

and Gonzalez,

95

1988). The amino acid ratios are:

Corbula 0.092 kO.019 (14) (minimum 0.065, maximum 0.128); Mactra 0.096 kO.029 (19) (minimum

0.053, maximum 0.168). Thus, the youngest shells analysed are from the southern barrier spits. Aminostratigraphy suggests that this unit started to accumulate penecontemporaneously with the barrier islands but it may have continued for longer, as is suggested by palaeontological evidence (Aguirre, 1990).

6. Discussion A model of changes in sea level for the late Quaternary of the northeastern Bonaerensian littoral area was recently published by Codignotto and Aguirre (1993) focused mainly on the last 6000 years. This is now supplemented by an interpretation that combines amino acid data with radiocarbon ages. The marine Pleistocene of the Bonaerensian littoral is restricted in comparison with the distribution of Holocene marine deposits. Geological and/or radiocarbon evidence suggests that outcrops corresponding with the ‘Last Interglacial’ (oxygen isotope sub-stage 5e) are found in Magdalena, Mar Chiquita and Puente de Pascua. Those at Magdalena, Mar Chiquita, Mar de1 Plata (Weiler et al., 1988; Isla, 1989; Fidalgo, 1979) are probably the same age. Because D/L ratios show interspecific variation the Pleistocene sediments included in this study were only those which contain species in common with the Holocene deposits analysed. It is now possible to propose a chronology: ( 1) The highest D/L ratios obtained for samples AC-091 1 (Magdalena) and A210 (Puente de Pascua) correspond with a pre-Holocene, higher than present sea-level stand, probably during oxygen isotope stage 5. This challenges the views of Isla, 1989, Fasano et al., 1984, and Weiler et al., 1988, who believed that this occurred during a “Wisconsinan interstadial”. The radiocarbon age estimates they used are close to the acceptable working limit of that method, and should be regarded as minimum ages only. (2) During the Holocene marine transgression, oceanic waters invaded the Bonaerensian coastal

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area and lie at an elevation of approximately i- 6 - 5 m above s.1. between ca. 7000 and 5000 yr B.P. The older deposits (location 1, Punta Indio at + 5 -6 m above s.1.) may have accumulated near the transgressive maximum at ca. 7000 yr B.P. (Fasano et al., 1987), probably at the peak of the Hypsithermal (between 7000 and 6000 yr B.P. in the southern hemisphere; Rabassa and Clapperton, 1990). This is supported by D/L ratios and the palaeoecology and palaeobiogeography of the entire molluscan fauna (Aguirre, 1993a). Similar ages for the Holocene Hypsithermal have been recorded for other areas of the southern hemisphere in Australia, New Zealand, southern Africa, the Argentinean plains (Burrows et al., 1977; Kendrick, 1976; Newsome and Pickett, 1993; Partridge, 1993; Avery, 1993; Iriondo and Garcia, 1993), and in the Northern Hemisphere (Chinzei et al., 1987; Lutaenko, 1993). In other areas, however, such as Tierra de1 Fuego, the molluscan species recorded from Holocene raised beaches (maximum sea level of + 8 - 10 m at ca. 6000 yr B.P.) are characteristic of cold shallow waters (Gordillo et al., 1993) and do not provide evidence of the climatic optimum. In addition, at locality 1 (7600-4460 yr B.P.) the different geometry of the beach ridges and the warmer aspect of their molluscan fauna, specifically the highest diversity and richness indices for these deposits (Aguirre, 1991, 1993b), suggest that they could have been deposited when the longshore sand transport was reversed. Perrier et al. (1992) identified periods of sea surface temperatures warmer than at present in Peruvian marine Holocene deposits related to El Niiio events, while Martin et al. (1993) recognized frequent similar ENSO-like events on the coast of Brazil before ca. 4000 yr B.P. Identification of such events in the Bonaerensian coastal area, however, requires more work. (3) The inner barrier islands (localities 5, 8 and 9) and the outer barrier islands (localities 3, 4 and 6) were deposited during successive sea-level stands of the post-Hypsithermal regression. The older barrier spits in the south (location 10 in the Mar Chiquita area) were deposited at approximately the same time as those at location 8 in Samborombon Bay, and after the formation of a

Geology 128 (1995) 85-104

palaeocape at Villa Gesell (Violante, 1988). The deposition of beach ridges in the south appears to have continued for longer, as is suggested by radiocarbon ages (Table 1) (Aguirre, 1993b). (4) A fall in temperature (and sea-level fall) occurred after ca. 5000 yr B.P., probably between 4500 and 4000 yr B.P, and was accompanied by a shift in wind direction and a change in oceanic circulation (including the direction and extent of shallow water currents). This is suggested by the less abundant warm water molluscan fauna, especially at locality 6, in comparison, for example, with localities 1 and 2 (Aguirre, 1993a, 1994) (see also Rabassa, 1987b). Data from other areas support this interpretation. In Japan, Chinzei et al. (1987) reported the highest Holocene temperature at ca. 7000 yr B.P., and a fall in temperature at ca. 4500-4000 yr B.P. In the western equatorial Pacific, Boltovskoy ( 1990) reported a temperature fall about 4000 yr B.P. In the central Brazilian coastal area Martin and Suguio (1992) established a sea-level fall by ca. 4000-3900 yr B.P., while in Tierra de1 Fuego, Gordillo et al. (1993) provided evidence for a sealevel fall at about 4000-4500 yr B.P. Elsewhere in the southern hemisphere several episodes of Holocene glacier re-advances occurred, one of them at about 4800-4500 yr B.P. (Burrows et al., 1976; Clapperton and Sugden, 1990).

Conclusions

(1) The amino acid age estimates of mollusca from late Quaternary shoreline deposits in northeastern Buenos Aires Province, Argentina, are in general agreement with the results of previous authors. (2) Although provisional, these dates provide new evidence for changing sea levels during the Holocene. The maximum sea level occurred about 7000 yr B.P., and its regression commenced ca. 5000 yr B.P. Evidence for a drop in sea level, and a fall in sea surface temperatures about 4500-4000 yr B.P. appears to correspond with a similar event elsewhere in both northern and southern hemispheres. (3) Older marine deposits are found at elevations

M.L. Aquirre et al./Marine

of + 5 - 6 m above m.s.1. in the Punta India-Punta Piedras area (localities 1 and 2). By combining the aminostratigraphical and palaeoecological evidence of the molluscan fauna it is possible to suggest that their marine deposits were deposited during the “climatic optimum” of the ‘Hypsithermal’ about 1000-5000 yr B.P. This was followed by at least one episode of lower sea surface temperatures. The younger deposits at Samborombon Bay and Mar Chiquita probably started to accumulate immediately after the deposition of Punta Indio-Punta Piedras deposits, but deposition at Samborombon Bay ended before that at Mar Chiquita.

Acknowledgements

We would like to thank Alfred0 Benialgo (Centro for his technical assistance; M.A. Gonzalez (Carl C: Zon Caldenius Foundation, Argentina) for providing the AC-09 11 and AC- 1014 samples and for discussion and useful comments on a previous manuscript; and Ester Farinati (Universidad National de1 SW, Argentina) and Anibal Figini (Latyr, Argentina) for supplying material for comparison. Three anonymous referees made useful suggestions which helped to improve the final version. This work was carried out during the tenure of an external fellowship from CONICET (National Council for Scientific and Technological Research of Argentina) to M.L.A. This is a contribution to IGCP Projects 274 and 28 1.

de Znvestiguciones Geolbgicas, CONICET)

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