Late Quaternary Deglacial History of the Mérida Andes, Venezuela. ND

JOURNAL OF QUATERNARY SCIENCE (in press) Copyright ß 2007 John Wiley & Sons, Ltd. Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jqs.1110

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Comment: Late Quaternary glacial erosion may result in elevated clastic sediment yields down-valley (e.g. Karleń, 1976; Leonard, 1986, 1997; Dirs- deglacial history of the Me´rida zowsky, 2004), such relationships may be unique to a particu- lar setting, and, in general, variations in sediment delivery may Andes, Venezuela result from a wider range of extrinsic and intrinsic, hydrologic, climatic, geomorphic or tectonic controls (Schumm, 1981). It is N. D. STANSELL, M. B. ABBOTT, P. J. worth noting that glacial retreat itself may lead to enhanced POLISSAR, A. P. WOLFE, M. BEZADA and sediment yield in the short term as freshly eroded materials V. RULL (2005) are released and/or exposed by the ice (Leonard, 1986, 1997), while previously deposited glacial sediments may be Journal of Quaternary Science 20: 801–812 subject to reworking and downstream transport for considerable periods of time following deglaciation (Church and Slay- maker, 1989). Despite the danger of exclusively relying on terrestrial deposits with limited age control, such deposits provide critical information for the proper interpretation of more continuous lake sediment records. Introduction Interpretations in Stansell et al. (2005) suffer from limited and/or faulty assessment of landforms within and adjacent to the Mucubajı´ valley, many of which have been previously In a recent paper in the Journal of Quaternary Science (20: documented. As discussed below, limitations also arise owing 801–812) Stansell et al. (2005), hereafter referred to as ‘the to incorrect interpretation of sedimentological evidence, espe- authors’, described the late Quaternary history of the cially from the upper Mucubajı´ catchment. Finally, minor Mucubajı´-Mucun˜uque valley in the northwestern Venezuelan omissions in Stansell et al. (2005) fail to give a fuller picture Andes, an important locality for ice age reconstructions first of just how the methods used were applied to the various recognised by Jahn and others (e.g. Jahn et al., 1925) and later analyses/interpretations. For example, Munsell colour is noted elucidated by Schubert (1974). In an attempt to reconstruct the as important in the methods, but no mention is made of colour palaeoclimatology of the area and especially its deglacial his- in the discussion, nor is any explanation offered as to how it tory, the authors analysed sediments (primarily cores) recov- may indicate glacial activity. With regard to organic content, ered from one lake and three bogs/palaeolakes in the the authors refer to loss on ignition (LOI) tests at 550C, but Mucubajı´ catchment plus six additional lakes in the region as over what time interval? Testing sediments for organic content proxy records of glacial activity. The authors claim their at temperatures as high as 550C is known to give erroneous results build upon previously published results from the region results, and according to NSSC standards 400C15hrÀ1 gives with the intent of refining the understanding of its glacial his- values considered much closer to actual organic matter content tory since the LGM. Because Figs 1 and 2 in Stansell et al. (NSSC, 1995). (2005) show few details of the valley under discussion and adjacent terrain, we present a more detailed map (Fig. 1) of the locality, highlighting the relationship between the Mucubajı´ and La Canãda drainages, and the prominent Mesa del Caballo (mislabelled in Stansell et al. (2005)), a Discussion large ridge landform comprised superficially of weathered pre-LGM till. Our map also shows the extent of glacial deposits associated with the LGM, including the four prominent Mucu- The authors’ reconstruction of glacial events in the Mucubajı´ bajı´ recessional moraines previously described by Salgado- valley begins with a depiction of LGM conditions during which Labouriau et al. (1977, 1992), Schubert (1974) and Mahaney glacier ice filled the valley to the prominent latero-frontal mor- and Kalm (1996). aine system situated 3500 m a.s.l. (Fig. 1). The duration of While the intent and results presented in Stansell et al. (2005) maximum ice extent is reported as 22 750 cal. yr BP to are commendable, and important in unravelling the palaeocli- 19 660 cal. yr BP (ca. 19 080–16 500 14C yr BP) based on out- matology and glacial history of the northern Venezuelan wash deposits and bracketing lacustrine sediments and peats, Andes, there are certain aspects of their interpretation and identified by Schubert and Rinaldi (1987) in the nearby La underlying assumptions that under closer scrutiny appear Canãda drainage basin (Fig. 1, Site 1). While the original inter- flawed. Limitations arise in the first place owing to assumptions pretation of these sediments (Schubert and Rinaldi, 1987) is made in correlating increased rates of inorganic lacustrine (or sound, application by Stansell et al. (2005) is complicated by bog) sedimentation and concomitant increases in magnetic the fact that the outwash site described is not adjacent to the susceptibility (MS) of these sediments with glacial activity in Mucubajı´ valley. Schubert and Rinaldi (1987) believed that the contributing basin. While it has long been recognised that meltwater entered the basin as seepage or via an unidentified JOURNAL OF QUATERNARY SCIENCE

Figure 1 Map of the Mucubajı´ area, northwestern Venezuelan Andes showing the correct position of Mesa del Caballo, the Pedregal basin, the recessional moraines above Laguna de Mucubajı´, fault strands, and sites discussed in the text: (1) Pedregal basin outwash (Schubert and Rinaldi, 1987); (2) recessional moraine bog (Stansell et al., 2005); (3) Mucubajı´ terrace (Salgado-Labouriau et al., 1977); (4) lateral moraine bog (Stansell et al., 2005)

breach in the El Caballo left lateral moraine (Fig. 1). Subse- either the El Caballo or Mucubajı´ valleys, subject to differences quent work, including fabric analysis, on a broader cross- in the timing of advance/retreat and meltwater production. section of La Canãda (also referred to as the Pedregal basin) Given the plethora of data collected and published on the deposits (Mahaney and Kalm, 1996; Mahaney et al., 2001, outwash ‘fan complex’ now exposed by the Quebrada La 2004; Dirszowsky et al., 2005) suggests that northward direc- Canãda, it is rather surprising that Stansell et al. (2005) interpret ted meltwater and sediment originated in the Mucuchache val- the basin to have derived its water volume and sediment load ley to the west, possibly through a moraine breach identified in from the Mucubajı´ valley some 2.5 km distant to the east. the field to the southwest of the basin. The 8 km long Mucu- Topographic relationships evident in the field and in Fig. 1 chache valley would have supported a much larger glacier than clearly indicate that meltwater discharge emanating from the

Mucubajı´ valley via the (breached) west side terminal moraine not glacial. The related suggestion that glacier retreat is could not have entered the La Canãda catchment for any length indicated by gradually decreasing clastic sedimentation of time. Instead, rapid downcutting lead to the formation of a (and MS) in Laguna de Mucubajı´ for a time leading up to deeply incised spillway directed north of the Mesa del Caballo, 8200 cal. yr BP (7380 14C yr BP) obviously depends on the and now occupied by Quebrada La Bejino´s (Fig. 1). previous interpretation of a readvance, and again on the Stansell et al. (2005) estimate principal retreat of ice from the assumption that inorganic sediments are glacially derived. lower Mucubajı´ valley to have occurred by ca. 15 730 cal. yr Unfortunately this interpretation does not account for the lack BP (13 270 14C yr BP) based on ‘incipient organic sedimenta- of any obvious increase in clastic sedimentation at depth that tion’ within a core obtained just up-valley of the uppermost might correlate with the proposed readvance itself, nor does recessional moraine (3650 m a.s.l.; Fig. 1, Site 2). It should it explain the very pronounced increase in clastic sedimenta- be noted that the dated organic layer (10 cm in thickness) tion just after 8200 cal. yr BP. The already low and decreasing occurs within the upper quarter of a 380 cm thick lacustrine inorganic content leading to 8200 cal. yr BP would seem to or glacio-lacustrine sediment package comprising thinly confirm that ice had remained up-valley of the recessional mor- bedded silts and sands (apparently deposited over till). Closely aine bog and terrace sites and was likely continuing to shrink related 14C-dated palaeolake sediments exposed in the nearby around this time. ‘Mucubajı´ terrace’ (Salgado-Labouriau et al., 1977; Mahaney Given the previous arguments, the authors’ claim that ice et al., 2007; Fig. 1, Site 3) and similar sediments in the La retreated up-valley of the recessional moraine bog and terrace Canãda basin (Mahaney et al., 2004; Dirszowsky et al., sites ca. 6280 cal. yr BP (5470 14C yr BP) is moot. The shift from 2005) indicate early postglacial lacustrine sedimentation rates inorganic lacustrine (not glacial) silts to peat in the recessional on the order of 0.35 cm yrÀ1, implying that glacier retreat from moraine bog site core at this time, once again, need not indi- the recessional moraine bog site (i.e. a moraine dammed lake) cate glacial activity. Climatically or hydrologically induced occurred somewhat earlier (ca. 900 yr) than the date reported lake-level fluctuations and eventual lake drainage as the reces- by Stansell et al. (2005). The occurrence of organic-rich clays sional moraine dam was cut down by outflow, readily explain and/or peat layers in these sediments may in part be a function the observed transition (and need to be accounted for in any of sediment influx and redistribution in the lake; however, it is interpretation of these materials). The related claim that glacier just as likely related to hydro-climatic (i.e. water balance) fluc- retreat from the recessional moraine sites coincides with tuations leading to temporary shallow water conditions (Dirs- another reduction in clastic sedimentation in Laguna de Mucu- zowsky et al., 2005). Therefore, while the Mucubajı´ glacier bajı´ ca. 6250 cal. yr BP (5455 14C yr BP) is also problematic, may have fluctuated up-valley of the recessional moraine bog since it would imply, according to the authors’ reasoning, sig- and terrace sites from 14 859 to 13 830 cal. yr BP (12 650 to nificant glacial advance during the preceding 2000 years 11 960 14C yr BP) as reported by Stansell et al. (2005), the var- (based on elevated clastic inputs ca. 7380–5455 14C yr BP). iations in peat versus lacustrine sediments (or organic content While the authors do not make this inference, neither do they in general) observed are not a reliable indication of glacial/ explain the elevated sediment inputs, which are most likely nonglacial activity in this case. related to tectonic/geomorphic processes in the catchment. The authors’ statement that the above recessional moraine The Mucubajı´ terrace site occurs about 80 m south (upstream) bog site sediments contain ‘no evidence of erosion by subse- of a conspicuous splay of the Bocono´ Fault system (Salgado- quent glacier readvances’ (Stansell et al., 2005: 809) is impor- Labouriau et al., 1977, 1992; Audemard et al., 1999; Mahaney tant considering their later interpretations of the upper core and et al., 2007) exhibiting vertical displacement of 6 m. Relative of additional bog sediments cored just down-valley of this site uplift of the up-valley surface and associated increases in on the outer flank of the prominent right lateral moraine (Fig. 1, stream gradient have led to down-cutting and remobilisation Site 4). Based on a transition from inorganic to organic (peat) of bog and palaeolake sediments comprising the terrace. While sediments near the base of their lateral moraine core, the it is uncertain at this time whether uplift/downcutting occurred authors propose a glacial readvance (to) and retreat from this as a single event or episodically, one 14C date from upper ter- down-valley position just prior to 9500 cal. yr BP (8500 14Cyr race level fluvial sediments (Mahaney et al., 2007) indicates BP). This interpretation would require an ice mass large enough that incision was under way some time ca. 6455 Æ 70 14Cyr to pass sediments over the moraine crest and perhaps as sub- BP (7370 Æ 70 cal. yr BP) (BGS-2245), coincident with stantial as that originally occupying the lower valley. Given increases in clastic sedimentation in Laguna de Mucubajı´ after the lack of disturbance (erosion or deformation) in the up-val- 7380 Æ 100 14C yr BP (8200 cal. yr BP). It is possible that sev- ley bog sediments described previously, the likelihood of such eral observed increases in clastic sedimentation and MS in a readvance would appear to be small. It is more likely that the Laguna de Mucubajı´ correspond to phases of fluvial incision inorganic materials (interbedded cobbles and coarse sand) at of these upper catchment sediments. the base of the authors’ 190 cm core simply represent a debris It stands to reason that if Late Pleistocene/early Holocene flow or other slope deposit (which no doubt limited coring), interpretations of deglaciation are incorrect, then later Holo- whereas the onset (or continuation) of peat accumulation cene interpretations based on the same evidence may be simi- may have been hydrologically or climatologically controlled larly faulty. Pollen records from the region have previously (i.e. reduced decomposition due to increased moisture and/or shown that vegetation and climate have remained similar to decreased temperature). today through most of the Holocene (Salgado-Labouriau The authors’ supporting evidence for a significant early et al., 1988, 1992) with cold events occurring ca. 6000– Holocene glacial readvance/retreat, based on lake sediments 5300 14C yr BP (‘La Caluta Cold/Dry Phase’; Salgado-Labouriau sampled down-valley (Laguna de Mucubajı´) and in an adjacent and Schubert, 1976) and in the 11th to 14th centuries (‘Piedras valley to the east (Laguna Negra) is similarly flawed. The Blancas Cold Phase’; Salgado-Labouriau, 1989), and relatively relatively sharp transition from thinly banded silts and sands warm conditions occurring ca. 2700–2500 14C yr BP (‘Miranda to banded organic clays in Laguna Negra (3473 m a.s.l.) Warm Phase’; Salgado-Labouriau et al., 1988). The second dated to ca. 10 000 cal. yr BP (8910 14C yr BP) may well indi- cold phase has been correlated with the Little Ice Age (Salgado- cate reduced sediment influx related to glacier retreat in the Labouriau, 1989; Rull et al., 1987) and with moraines present catchment, but it says nothing about the elevation reached at upper elevations of the Sierra Nevada de Me´rida (Humboldt by the ice—the inorganic sediments in their core are lacustrine massif) 40 km to the southwest (Schubert, 1972, 1998).

While there is an apparent correlation between the two palyno- sion in light of what is discussed here and in other logically inferred cold events and elevated clastic sedimenta- publications that were apparently overlooked as the authors tion and MS in Laguna de Mucubajı´, it should be emphasised prepared their results and interpretation. that most clastic sedimentation/MS peaks in this record occur during ‘average’ climatic conditions (i.e. ca. 7400–6000 14Cyr BP and ca. 4100–2700 14C yr BP), and in one case (ca. 2600 14C yr BP) during a known warm period. Considering References the uninterrupted records of peat (non-glacial) accumulation reported by Stansell et al. (2005) from the Pa´ramo de Piedras Blancas and Pa´ramo el Banco (16 km to the northwest of Audemard F, Pantosti D, Machette M, Costa C, Okumura K, Cowan H, Diederix H, Ferrer C, participants of the South American Field Work- Laguna de Mucubajı´ and ranging from 4170 to 4366 m a.s.l.) 14 shop on Paleoseismology. 1999. Trench investigations along the since ca. 15 010–9360 cal. yr BP (12 740–8350 C yr BP) and Me´rida section of the Bocono´ fault (central Venezuelan Andes), indicating glacial retreat/disappearance there, it would seem Venezuela. Tectonophysics 308: 1–21. unlikely that ice advanced substantially below these elevations Church M, Slaymaker O. 1989. Disequilibrium of Holocene sediment in the Mucubajı´ valley during the Holocene. yield in glaciated British Columbia. Nature 337: 452–454. The authors’ reference to ‘Little Ice Age moraines in the Dirszowsky RW. 2004. Bed sediment sources and mixing in the gla- Mucubajı´ cirque’, based presumably on Polissar et al. (2006), cierized upper Fraser River watershed, British Columbia. Earth Sur- and their further assertion that this ‘indicates the sensitivity of face Processes and Landforms 29: 533–552. this catchment to Holocene glaciation’ are particularly Dirszowsky RW, Mahaney WC, Hodder KR, Milner MW, Kalm V, astounding and reveal a lack of familiarity with the physical Bezada M, Beukens RP. 2005. Lithostratigraphy of the Me´rida (Wis- consinan) glaciation and Pedregal interstade, Me´rida Andes, north- environment of the Mucubajı´ catchment, since no LIA mor- western Venezuela. Journal of South American Earth Sciences 19: aines exist there (reconnaissances by Mahaney and Kalm 525–536. between 1994 and 2001). Presumably we (Mahaney and Kalm) Jahn A. 1925. Observacions glaciologicas en los Andes venezolanos. might have inadvertently overlooked some evidence of LIA gla- Cultura Venezolana 8: 265–280. cier oscillations in the upper Mucubajı´ valley, but our logs Karleń W. 1976. Lacustrine sediments and tree limit variations as indi- show only bedrock, associated avalanche couloirs and mass- cators of Holocene climatic fluctuations in Lappland, northern wasted debris below the high ridge of Pico Mucun˜uque Sweden. Geografiska Annaler A 58: 1–34. (4609 m a.s.l.), and valley floor surfaces above 3800 m Leonard EM. 1986. Use of lacustrine sedimentary sequences as indica- a.s.l. laced with a variety of thin discontinuous glacial meltout tors of Holocene glacial history, Banff National Park, Alberta, sediments, very inconsequential ground moraine, fluvial sedi- Canada. Quaternary Research 26: 218–231. Leonard EM. 1997. The relationship between glacial activity and sediments and talus. ment production: evidence from a 4450-year varve record of Neo- glacial sedimentation in Hector Lake, Alberta. Journal of Paleolimnology 17: 319–330. Mahaney WC, Kalm V (eds). 1996. Field Guide for the International Conference on Quaternary Glaciation and Paleoclimate in the Andes Conclusions Mountains, June 21–July 1, 1996. Quaternary Surveys Ltd.: Toronto. Mahaney WC, Russell SE, Milner MW, Kalm V, Bezada M, Hancock RGV, Beukens RP. 2001. Paleopedology of Middle Wisconsin/ It appears that considerable disagreement exists between the Weichselian paleosols in the Me´rida Andes, Venezuela. Geoderma palaeoenvironmental interpretations of Stansell et al. (2005) 104: 215–237. Mahaney WC, Dirszowsky RW, Milner MW, Menzies J, Stewart A, and various work carried out by Mahaney, Kalm, Dirszowsky Kalm V, Bezada M. 2004. Quartz microtextures and microstructures and others in the same setting. Overall, the lack of terrestrial owing to deformation of glaciolacustrine sediments in the northern (moraine) evidence of glacial activity in the uppermost Mucu- Venezuelan Andes. Journal of Quaternary Science 19: 23–33. bajı´ catchment following initial deglaciation and the deposi- Mahaney WC, Dirszowsky RD, Milner MW, Harmsen R, Finkelstein S, tion of cross-valley moraines up to 3800 m a.s.l., the Kalm V, Bezada M, Hancock RGV. 2007. Soil stratigraphy and eco- preservation without deformation of Late Pleistocene lacus- logical relationships of a Late Glacial–Holocene fluvial terrace trine/glacio-lacustrine deposits, and the availability of alterna- sequence, Sierra Nevada National Park, Northern Venezuelan tive explanations for the observed variations in organic versus Andes. Journal of South American Earth Sciences 23: 46–60. inorganic sedimentation in lakes, palaeolakes and bogs, argue NSSC (National Soil Survey Center). 1995. Soil Survey Laboratory against the authors’ belief that ice persisted in the catchment Information Manual. Soil Survey Investigations Report No. 45. Ver- sion 1.0, May, 1995, USDA: Washington, DC. and/or readvanced down-valley long after the Pleistocene/ Polissar PJ, Abbott MB, Wolfe AP, Bezada M, Rull V, Bradley RS. Holocene transition or Younger Dryas climatic event. Consid- 2006. Solar modulation of Little Ice Age climate in the tropical ering also, relatively complacent pollen signals in the catch- Andes. Proceedings of the National Academy of Science 103: ment, and uninterrupted records of peat accumulation at 8937–8942. nearby Pa´ramo de Piedras Blancas and Pa´ramo el Bianco sites Rull V. Salgado-Labouriau ML, Schubert C, Valastro S. 1987. Late (Stansell et al., 2005), Holocene glacial activity in the Mucu- Holocene temperature depression in the Venezuelan Andes: palyno- bajı´ valley would appear to be overstated. logical evidence. Palaeogeography, Palaeoclimatology, Palaeoecol- Over-reliance on archival material gleaned from sediment ogy 60: 109–121. cores may lead to serious misinterpretation of proxy indicators Salgado-Labouriau ML. 1989. Late Quaternary climate oscillations in in connection with climate or environmental change. Examples the Venezuelan Andes. Biological International 18: 12–14. Salgado-Labouriau ML, Schubert C. 1976. Palynology of Holocene of such over-reliance abound in Stansell et al. (2005), espe- peat bogs from the central Venezuelan Andes. Palaeogeography, cially when one considers extrapolations made into the high Palaeoclimatology, Palaeoecology 19: 147–156. Mucubajı´ valley that are at odds with the known landform Salgado-Labouriau ML, Schubert C, Valastro S. 1977. Paleoecologic distribution. While we welcome the new data provided by analysis of a Late-Quaternary terrace from Mucubajı´, Venezuelan Stansell et al. (2005) pertaining to the palaeoclimatology of Andes. Journal of Biogeography 4: 313–325. the Venezuelan Andes, it is evident that the glacial geologi- Salgado-Labouriau ML, Rull V, Schubert C, Valastro S. 1988. The estab- cal/geomorphological interpretations made therein need revi- lishment of vegetation after Late Pleistocene deglaciation in the

Pa´ramo de Miranda, Venezuelan Andes. Review of Palaeobotany WILLIAM C. MAHANEY and Palynology 55: 5–17. Geomorphology and Pedology Laboratory Salgado-Labouriau ML, Bradley RS, Yuretich R, Weingarten B. 1992. York University Paleoecological analysis of the sediments of Lake Mucubajı´, Vene- 4700 Keele Street zuelan Andes. Journal of Biogeography 19: 317–327. North York Schubert C. 1972. Geomorphology and glacier retreat in the Pico Boli- var area, Sierra Nevada de Me´rida, Venezuela. Zeitschrift fu¨r Ontario M3J 1P3 Gletscherkunde und Glazialgeologie 8: 189–202. Canada Schubert C. 1974. Late Pleistocene Me´rida Glaciation, Venezuelan Andes. Boreas 3: 147–152. RANDY W. DIRSZOWSKY Schubert C. 1998. Glaciers of Venezuela. In Satellite Image Atlas of Department of Geography and Department of Biology Glaciers of the World: Glaciers of South America, Williams RS, Fer- Laurentian University rigno JG (eds). Professional Paper no. 1386-I. US Geological Survey: 935 Ramsey Lake Road Washington, DC. Schubert C, Rinaldi M. 1987. Neuvos datos sobre la cronologıá del Sudbury estadio tardıó de la GlaciaciońMe´rida, Andes Venezolanos. Acta Ontario P3E 2C6 Cientı´fica Venezolana 38: 135–136. Canada Schumm SA. 1981. Evolution and response of the fluvial system: sedimentological implications. In Recent and Ancient Non-marine VOLLI KALM Depositional Environments: Models for Exploration, Etheridge FG, Flores RM (eds). SEPM Special Publication 31, Society of Economic Institute of Geology Palaeontologists and Mineralogists, Tulsa, Oklahoma; 19–29. Tartu University Stansell ND, Abbott MB, Polissar J, Wolfe AP, Bezada M, Rull V. 2005. Vanemuise Str. 46 Late Quaternary deglacial history of the Me´rida Andes, Venezuela. EE51014 Tartu Journal of Quaternary Science 20: 801–812. Estonia

Copyright ß 2007 John Wiley & Sons, Ltd. J. Quaternary Sci., (in press) DOI: 10.1002/jqs JOURNAL OF QUATERNARY SCIENCE (in press) Copyright ß 2007 John Wiley & Sons, Ltd. Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jqs.1109

Correspondence

the terminal and lateral moraines of the El Caballo and Reply: Late Quaternary deglacial La Mucuchache glaciers. A chemically untreated organic history of the Me´rida Andes, sample from 1.5 m above the base of the section yielded a conventional age of 22 750 cal. yr BP (19 000 14C yr BP), Venezuela: response whereas a peat sample immediately below the top of the section yielded an age of 19 960 cal. yr BP (16 500 14C yr BP). to comment We recognise that other workers (e.g. Mahaney et al., 2001, 2004; Dirszowsky et al., 2005) have re-analysed the Pedregal fan complex. For example, the sedimentology was described in detail by Dirszowsky et al. (2005). However, the chronology in We welcome the opportunity to respond to Mahaney et al.’s these papers is questionable and not useful for constraining the critique of our article on the deglacial history of the LGM. All of the recently published dates are on refractory Venezuelan Andes (Stansell et al., 2005). The source of our organic matter isolated using a cellulose extraction technique disagreement appears to stem primarily from differences in which concentrates refractory carbon, resulting in ages research strategies and methodology. Our work in the between 47 000 and 60 000 14C yr BP. These ages include mul- Venezuelan Andes employs lake sediment archives that record tiple reversals and are at the very edge of interpretability as continuous sedimentary sequences and can be dated using finite radiocarbon ages; nonetheless, the authors have inter- accelerator mass spectrometry (AMS) radiocarbon dating of preted them as valid ages. Similar extraction methods for isolat- identifiable terrestrial macrofossils. Mahaney et al. focus on ing the most refractory organic matter have been shown to temporally discontinuous deposits (moraines and glacial– produce ages that are much older than the age of deposition fluvial sediments) and radiocarbon dates on refractory organic (Abbott and Stafford, 1996). Mahaney et al. (2001) described matter that are most likely to yield anomalously old results. The and dated palaeosols in the region; however, these data are comment of Mahaney et al. is fraught with serious misunder- not relevant to this discussion because the ages presented standings of the data we presented, and is thus misleading to are all between 48 000 and > 64 000 14C yr BP and predate readers who have not examined the original data. In our reply, the LGM by tens of millennia. Mahaney et al. (2004) allude we specifically address their claims regarding: (1) the local to the LGM without refining the chronology of its local expres- timing of the Last Glacial Maximum; (2) the chronology of sion, and we neither cited this paper in our analysis from lake the late Pleistocene–Holocene transition; and (3) Holocene sediments, nor have reason to believe that the original dates for environmental variability. the LGM reported by Schubert and Rinaldi (1987) have been Lakes in previously glaciated catchments are excellent refined by their recent analyses. recorders of glacial variability because these systems commonly preserve uninterrupted glacio-lacustrine sequences, and contain in situ organic matter that can be isolated for 14C dating. Lake sediment cores can also be analysed at high Late Pleistocene–Holocene transition stratigraphic resolution for an array of sedimentological para- meters, which in turn can be used to infer environmental changes over a range of timescales. Lithologic transitions in Our interpretation of Late Pleistocene and Holocene glacial peat bogs that either formed within or directly adjacent to activity in the Mucubajı´ valley is based on multiple sedimen- moraines can also date significant movements of a glacier tary records, including a recessional moraine bog, a lateral margin. Combined, these types of records are advantageous moraine bog, a continuous record from Laguna Mucubajı´, for dating glacial activity because they are less susceptible to and palaeoenvironmental evidence from the Mucubajı´ Terrace surface processes such as erosion and weathering, and provide (Salgado-Labouriau et al., 1977). Our methods utilised multiple broadened perspectives on the attendant climatic conditions proxies, including magnetic susceptibility profiles (Seltzer prior to, during, and after a given glacial advance. et al., 2002) and loss-on-ignition (LOI) measured according to standard laboratory protocols (Dean, 1974; Heiri et al., 2001; Boyle, 2004). LOI data were verified against oxidative elemental analysis of organic carbon content. The base of a Local timing of the Last 10 cm section of lacustrine sediments from the Mucubajı´ Glacial Maximum (LGM) recessional moraine bog site dates to 15 730 (15 520 to 15 920) cal. yr BP, and provides a minimum limiting age for ice being restricted far up-valley from its LGM position. The Schubert and Rinaldi (1987) radiocarbon-dated a presumed terrace site of Salgado-Labouriau et al. (1977) is down-valley LGM sedimentary sequence at the southern margin of Mesa of the recessional moraine bog site and contains alternating del Caballo in the Pedregal fan complex. They interpreted sequences of interbedded peat, clays and glaciofluvial the sequence as part of an outwash plain that extended from sediments. The lower ~2.6 m of the 5 m sequence dates JOURNAL OF QUATERNARY SCIENCE between 14 880 and 13 830 cal. yr BP (12 650 and 11 960 14C We note that these results are directly compatible with the yr BP; Salgado-Labouriau et al., 1977), while the sediment ages for deglaciation obtained in other regions of the northern lithology implies multiple glacier advances and retreats during tropics, including the highlands of Mexico (Vazquez-Selem this interval. We emphasise that the Mucubajı´ terrace sequence and Heine, 2004) and Costa Rica (Orvis and Horn, 2000). of Salgado-Labouriau et al. (1977) shows absolutely no Thus, they add considerable new data to debates concerning evidence of glacial activity after 13 830 cal. yr BP, in contrast the timing of tropical deglaciation, in relation to the high to the bog site reported by Stansell et al. (2005). This does not northern latitudes (Clark, 2002). result in a contradiction of findings because the upper 2.6 m of the Mucubajı´ terrace was not dated by Salgado-Labouriau et al. (1977) and does not appear to preserve the upper sequence of Holocene age. The ~900 yr earlier age of glacial retreat Holocene climate and glacial variability proposed in the comment by Mahaney et al. is based on unpub- lished data that extrapolates deglaciation ages from dated horizons using average ‘early postglacial’ sediment accumula- Climate variability in Venezuela during the Late Pleistocene tion rates. The high clastic input during and following glacial and Holocene has been highly variable as documented by a retreat is likely to vary dramatically from site to site, implying series of studies (Bradbury et al., 1981; Salgado-Labouriau, that any extrapolation based upon ‘average’ rates is likely to be, 1984; Bradley et al., 1985; Rull, 1998; Curtis et al., 1999; Haug at best, an educated guess. Regardless, a difference in 900 yr et al., 2001; Rull et al., 2005; Polissar et al., 2006a). However, for the age of deglaciation does not affect the principal findings many of these studies have used pollen stratigraphic analyses at in our paper. We also strongly emphasise that only tentative low temporal resolution, making it difficult to compare them evidence of a Younger Dryas equivalent exists from terrestrial directly to the Mucubajı´ sedimentological record. It is note- records in the Venezuelan Andes (Salgado-Labouriau, 1989; worthy that inorganic sediment content in Laguna Mucubajı´ Weingarten et al., 1991), nor is it conclusive that a widespread is low just prior to 8200 cal. yr BP, after which the values cooling event took place in the region during that time increase until 6250 cal. yr BP. Furthermore there is also an (e.g. Rull et al., 2005). abrupt transition from inorganic silt to peat at 6280 cal. yr BP Our interpretation of the timing of deglaciation in the in the Mucubajı´ recessional moraine bog core. The coincident Mucubajı´ valley is corroborated by data obtained elsewhere timing of major transitions at both sites suggests that ice was in the Venezuelan Andes. Radiocarbon-dated deglacial restricted to the uppermost reaches of the catchment by ca. magnetic susceptibility profiles from lake sediments in the 6300 cal. yr BP. Pa´ramo de Piedras Blancas, the Pa´ramo el Banco, and Laguna The published pollen records from the Venezuelan Andes Negra (adjacent valley to Mucubajı´) indicate at least two provide, at best, century-scale integrations of ecosystem devel- phases of widespread glacial recession at the end of the Pleis- opment and climate oscillations; short-duration events (e.g. tocene. The first phase occurred immediately prior to 15 010 sub-century-scale) cannot be identified. Despite these caveats, and 14 250 cal. yr BP, as recorded in Laguna Verde Alta ‘the known warm period’ in the pollen record between ca. (4215 m) and Laguna Verde Baja (4170 m), respectively. The 2800 and 2550 cal. yr BP (2690 to 2500 14C BP; Salgado- second phase of deglaciation is recorded in the western Labouriau et al., 1988) indeed corresponds to an overall Paramo de Piedras Blancas and in the Paramo El Banco, where decrease in clastic sediment content in Laguna Mucubajı´, there is a sharp transition from glaciolacustrine to postglacial punctuated by an abrupt and short-lived increase in sediment organic-rich lake sedimentation in three uninterrupted records. around 2760 cal. yr BP. Likewise, the Mucubajı´ record pre- The transition at Laguna Los Locos (4366 m) dates to serves evidence for at least four short-lived glacial advances 9650 cal. yr BP. The transition at Laguna Grande de Los Patos during the last 400 years, which correspond to the pronounced (4185 m) dates to 10 000 cal. yr BP, and the transition at Laguna local expression of the Little Ice Age (Polissar et al., 2006b). It is La Posita (4228 m) dates to 9360 cal. yr BP. thus clear that, while broad correspondence between the paly- The highest elevations in the southeast-facing Paramo El nological and sedimentological records exists, only the latter is Banco and Paramo de Piedras Blancas are about 300 m lower at present sufficiently resolved to identify the numerous rapid than the headwall of the Mucubajı´ valley (4609 m). It is therefore changes that appear to punctuate the Late Holocene glacial possible that glaciers survived longer in the Mucubajı´ and climate history of the Mucubajı´ valley. valley relative to the El Banco and Piedras Blancas paramos. In The role of seismicity in limnological systems should always addition, precipitation and cloud cover are much reduced on the be considered; however, the claim that tectonic activity caused slopes of the Paramo El Banco and Piedras Blancas relative to the observed sedimentological changes in Laguna Mucubajı´ is the Mucubajı´ region. Therefore the geographic setting of not supported by the data. Mahaney et al.’s single radiocarbon these slopes probably drove a regional asymmetry in the date bearing on the region’s seismic history remains unpub- elevation of glaciers (e.g. Hastenrath, 1985), and may have con- lished, so that, without adequate details on the sample’s exact tributed to a deglacial lag for the higher, northwest-facing stratigraphic context, no meaningful inferences concerning its regions. relevance can be made at this time. Moreover, the available The Laguna Negra site, adjacent to the Mucubajı´ valley, evidence indicates that a strong statistical relationship exists provides supporting evidence for restricted ice in the region between climate change and the observed Mucubajı´ sedimen- at the onset of the Holocene. Laguna Negra (3473 m) is situated tological record (Polissar, 2005). at a low elevation relative to the valley headwall (4609 m), and More research using multidisciplinary approaches is still records a lithological deglacial transition at 10 040 cal. yr BP. needed in order to refine the timing of cooling and warming We agree that the change in sedimentation at Laguna Negra events in the northern tropics. Sites with continuous, high-reso- does not necessarily imply that the entire catchment was ice- lution records should be targeted for future fieldwork and free, but it strongly suggests that ice was, at best, restricted to improved age control should be emphasised. The evidence the highest elevations of the valley. Likewise, the Mucubajı´ lat- gathered to date suggests that the terminal Pleistocene degla- eral moraine bog indicates that up-valley glacier retreat was ciation of the Northern Hemisphere tropics occurred later than almost complete by 10 000 cal. yr BP. A core from this site in the Southern Hemisphere, and that multiple cooling events preserves till capped by peat dating to 9500 cal. yr BP. took place in the Venezuelan Andes during the late Holocene,

Copyright ß 2007 John Wiley & Sons, Ltd. J. Quaternary Sci., (in press) DOI: 10.1002/jqs CORRESPONDENCE when alpine glaciers developed anew in the highest catch- sedimentation rates and glacier retreat. Review of Palaeobotany ments. Lively discussion is an essential component of any and Palynology 99: 95–114. scientific enterprise. We value the different views of Mahaney Rull V, Abbott MB, Polissar PJ, Wolfe AP, Bezada M, Bradley RS. 2005. et al. and welcome this opportunity to defend our methods and 15,000-yr pollen record of vegetation change in the high altitude tropical Andes at Laguna Verde Alta, Venezuela. Quaternary Research results. We hope this exchange will both educate readers 64: 308–317. and further the scientific goal of understanding the timing Salgado-Labouriau ML. 1984. Late-Quaternary palynological studies in and causes of environmental change. the Venezuelan Andes. Erdwissenschaftliche Forschung 18: 279– 293. Salgado-Labouriau ML. 1989. 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