Hostname: page-component-78c5997874-ndw9j Total loading time: 0 Render date: 2024-11-06T04:17:52.274Z Has data issue: false hasContentIssue false
  • English
  • Français

Holocene vegetation and climate change recorded in alpine bog sediments from the Borreguiles de la Virgen, Sierra Nevada, southern Spain

Published online by Cambridge University Press:  20 January 2017

Gonzalo Jiménez-Moreno  and
R. Scott Anderson
Gonzalo Jiménez-Moreno*
Affiliation:
Departamento de Estratigrafía y Paleontología, Universidad de Granada, Fuente Nueva s/n, 18002, Granada, Spain
R. Scott Anderson
Affiliation:
School of Earth Sciences & Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011, USA
*
*Corresponding author. Fax: + 34 958 248528. E-mail address:[email protected] (G. Jiménez-Moreno).
Get access Rights & Permissions [Opens in a new window]

Abstract

High-resolution pollen and magnetic susceptibility (MS) analyses have been carried out on a sediment core taken from a high-elevation alpine bog area located in Sierra Nevada, southern Spain. The earliest part of the record, from 8200 to about 7000 cal yr BP, is characterized by the highest abundance of arboreal pollen and Pediastrum, indicating the warmest and wettest conditions in the area at that time. The pollen record shows a progressive aridification since 7000 cal yr BP that occurred in two steps, first shown by a decrease in Pinus, replaced by Poaceae from 7000 to 4600 cal yr BP and then by Cyperaceae, Artemisia and Amaranthaceae from 4600 to 1200 cal yr BP. Pediastrum also decreased progressively and totally disappeared at ca. 3000 yr ago. The progressive aridification is punctuated by periodically enhanced drought at ca. 6500, 5200 and 4000 cal yr BP that coincide in timing and duration with well-known dry events in the Mediterranean and other areas. Since 1200 cal yr BP, several changes are observed in the vegetation that probably indicate the high-impact of humans in the Sierra Nevada, with pasturing leading to nutrient enrichment and eutrophication of the bog, Pinus reforestation and Olea cultivation at lower elevations.

Keywords

PollenVegetationClimateAridificationMediterranean areaHolocene
Type
Original Articles
Information
Quaternary Research , Volume 77 , Issue 1 , January 2012 , pp. 44 - 53
Copyright
University of Washington

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Ahmed, S.E., Cain, R.F., (1972). Revision of the genera Sporormia and Sporormiella . Canadian Journal of Botany 50, 419–477.Google Scholar
Anderson, R.S., Jiménez-Moreno, G., Carrión, J.S., Pérez-Martínez, C., (2011). Holocene vegetation history from Laguna de Río Seco, Sierra Nevada, southern Spain. Quaternary Science Reviews 30, 1615–1629.Google Scholar
Andrade, A., Valdeolmillos, A., Ruíz-Zapata, B., (1994). Modern pollen spectra and contemporary vegetation in the Paramera Mountain range (Ávila, Spain). Review of Palaeobotany and Palynology 82, 127–139.Google Scholar
Arévalo Barroso, A., (1992). Atlas Nacional de España, Sección II, Grupo 9, Climatología, Ministerio de Obras Públicas y Transportes, Dirección General del Instituto Geográfico Nacional, Madrid.Google Scholar
Arias Abellán, J.A., (1981). La repoblación forestal en la vertiente norte de Sierra Nevada. Cuadernos geográficos de la Universidad de Granada. 283–305.Google Scholar
Arz, H.W., Lamy, F., Pátzold, J., (2006). A pronounced dry event recorded around 4.2 ka in brine sediments from the northern Red Sea. Quaternary Research 66, 432–441.CrossRefGoogle Scholar
Asioli, A., Medioli, F.S., Patterson, R.T., (1996). Thecamoebians as a tool for reconstruction of paleoenvironments in some Italian lakes in the foothills of the southern Alps (Orta, Varese and Candia). Journal of Foraminiferal Research 26, 248–261.CrossRefGoogle Scholar
Bar-Matthews, M., Ayalon, A., Kaufmann, A., (2000). Timing and hydrological conditions of sapropel events in the eastern Mediterranean, as evident from speleothems, Soreq Cave, Israel. Chemical Geology 169, 145–156.CrossRefGoogle Scholar
Booth, R.K., Jackson, S.T., Forman, S.L., Kutzbach, J.E., Bettis III, E.A., Kreigs, J., Wright, D.K., (2005). A severe centennial-scale drought in midcontinental North America 4200 years ago and apparent global linkages. The Holocene 15, 321–328.Google Scholar
Boudreau, E.A., Galloway, J.M., Patterson, R.T., Kumar, A., Michel, F.A., (2005). A paleolimnological record of Holocene climate and environmental change in the Temagami region, northeastern Ontario. Journal of Paleolimnology 33, 445–461.Google Scholar
Brayshaw, D.J., Rambeau, C.M.C., Smith, S.J., (2011). Changes in Mediterranean climate during the Holocene: insights from global and regional climate modelling. The Holocene 21, 15–31.CrossRefGoogle Scholar
Broström, A., Coe, M., Harrison, S., Gallimore, R., Kutzbach, J.E., Foley, J., Prentice, I.C., Behling, P., (1998). Land surface feedbacks and palaeomonsoons in northern Africa. Geophysical Research Letters 25, 3615–3618.Google Scholar
Burjachs, F., Giralt, S., Roca, J.R., Seret, G., Julià, R., (1997). Palinología holocénica y desertización en el Mediterráneo occidental. Ibáñez, J.J., Valero, B.L., Machado, C., El paisaje mediterráneo a travás del espacio y del tiempo. Implicaciones en la desertificación. Geoforma Editores, Logroño. 379–394.Google Scholar
Cacho, I., Grimalt, J.O., Canals, M., (2002). Response of the Western Mediterranean Sea to rapid climatic variability during the last 50,000 years: a molecular biomarker approach. Journal of Marine Systems 33–34, 253–272.CrossRefGoogle Scholar
Carrión, J.S., (2002). Patterns and processes of Late Quaternary environmental change in a montane region of southwestern Europe. Quaternary Science Reviews 21, 2047–2066.Google Scholar
Carrión, J.S., Munuera, M., Dupr", M., Andrade, A., (2001). Abrupt vegetation changes in the Segura mountains of southern Spain throughout the Holocene. Journal of Ecology 89, 783–797.Google Scholar
Carrión, J.S., Fuentes, N., González-Sampériz, P., Sánchez Quirante, L., Finlayson, J.C., Fernández, S., Andrade, A., (2007). Holocene environmental change in a montane region of sourthern Europe with a long history of human settlement. Quaternary Science Reviews 26, 1455–1475.CrossRefGoogle Scholar
Carrión, J.S., Fernández, S., González-Sampériz, P., Gil-Romera, G., Badal, E., Carrión-Marco, Y., L"pez-Merino, L., López-Sáez, J.A., Fierro, E., Burjachs, F., (2010). Expected trends and surprises in the Lateglacial and Holocene vegetation history of the Iberian Peninsula and Balearic Islands. Review of Palaeobotany and Palynology 162, 458–476.CrossRefGoogle Scholar
Castillo Martín, A., (2009). Lagunas de Sierra Nevada. Editorial Universidad de Granada, Granada.Google Scholar
Cheddadi, R., Yu, G., Guiot, J., Harrison, S.P., Prentice, I.C., (1997). The climate 6000 years ago in Europe. Climate Dynamics 13, 1–9.CrossRefGoogle Scholar
deMenocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarnthein, M., Baker, L., Yarusinsky, M., (2000). Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing. Quaternary Science Reviews 19, 347–361.CrossRefGoogle Scholar
Dresch, J., (1937). De la Serra Nevada au Grand Atlas, formes glaciaires et formes de nivation. M"langes de G"ographie et d'Orientalisme offerts a E.F. Gautier. Tours. 194–212.Google Scholar
Drysdale, R., Zanchetta, G., Hellstrom, J., Maas, R., Fallick, A., Cartwright, I., Piccini, L., Pickett, M., (2006). Late Holocene drought responsible for the collapse of Old World civilizations is recorded in an Italian cave flowstone. Geology 34, 101–104.Google Scholar
El Aallali, A., L"pez Nieto, J.M., P"rez Raya, F., Molero Mesa, J., (1998). Estudio de la vegetación forestal en la vertiente sur de Sierra Nevada (Alpujarra Alta granadina). Ininera Geobotanica 11, 387–402.Google Scholar
Emeis, K.-C., Struck, U., Schulz, H.-M., Rosenberg, R., Bernasconi, S., Erlekeuser, H., Sakamoto, T., Martinez-Ruiz, F., (2000). Temperature and salinity variations of Mediterranean Sea surface waters over the last 16,000 years from records of planktonic stable oxygen isotopes and alkenone unsaturation ratios. Palaeogeography, Palaeoclimatology, Palaeoecology 158, 259–280.Google Scholar
Escobar, J., Brenner, M., Whitmore, T.J., Kenney, W.F., Curtis, J.H., (2008). Ecology of testate amoebae (thecamoebians) in subtropical Florida lakes. Journal of Paleolimnology 40, 715–731.Google Scholar
Faegri, K., Iversen, J., (1989). Textbook of Pollen Analysis. Wiley, New York.Google Scholar
Fletcher, W.J., Sanchez Goñi, M.F., (2008). Orbital- and sub-orbital-scale climate impacts on vegetation of the western Mediterranean basin over the last 48,000 yr. Quaternary Research 70, 451–464.Google Scholar
Fletcher, W., Boski, T., Moura, D., (2007). Palynological evidence for environmental and climatic change in the lower Guadiana valley (Portugal) during the last 13,000 years. The Holocene 17, 479–492.Google Scholar
Fletcher, W.J., Sanchez Goñi, M.F., Peyron, O., Dormoy, I., (2010). Abrupt climate changes of the last deglaciation detected in a Western Mediterranean forest record. Climates of the Past 6, 245–264.Google Scholar
Frigola, J., Moreno, A., Cacho, I., Canals, M., Sierro, F.J., Flores, J.A., Grimalt, J.O., Hodell, D.A., Curtis, J.H., (2007). Holocene climate variability in the western Mediterranean region from a deepwater sediment record. Paleoceanography 22, PA2209.Google Scholar
Gasse, F., (2002). Diatom-inferred salinity and carbonate oxygen isotopes in Holocene waterbodies of the western Sahara and Sahel (Africa). Quaternary Science Reviews 21, 737–767.CrossRefGoogle Scholar
Gill, J.L., Williams, J.W., Jackson, S.T., Lininger, K.B., Robinson, G.S., (2009). Pleistocene megafaunal collapse, novel plant communities, and enhanced fire regimes in North America. Science 326, 1100–1103.Google Scholar
Giraudi, C., Magny, M., Zanchetta, G., Drysdale, R.N., (2011). The Holocene climatic evolution of Mediterranean Italy: a review of the continental geological data. The Holocene 21, 105–115.CrossRefGoogle Scholar
Gómez Ortiz, A., (1987). Morfologia glaciar en la vertiente meridional de Sierra Nevada (area Veleta-Mulhacen). Estudios Geográficos 48, 188, 379–407.Google Scholar
Gómez Ortiz, A., Schulte, L., Salvador Franch, F., Sánchez Gómez, S., Simón Torres, M., (2001). Glacial and Periglacial Geomorhology of Sierra Nevada (Spain). University of Barcelona, Barcelona, Spain.Google Scholar
Gó Ortiz, A., Schulte, L., Salvador Franch, F., Palacios Estremera, D., Sanjos" Blasco, J.J., Atkinson Gordo, A., (2004). Deglaciación reciente de Sierra Nevada. Repercusiones morfogénicas, nuevos datos y perspectivas de estudio futuro. Cuadernos de Investigación Geográfica 30, 147–168.Google Scholar
Gó Ortiz, A., Schulte, L., Salvador Franch, F., Palacios Estremera, D., Sanz de Galdeano, C., Sanjosé Blasco, J.J., Tanarro García, L.M., Atkinson, A., (2005). The Geomorphological Unity of the Veleta: A Particular Area of the Sierra Nevada. Guidebook, Sixth International Conference on Geomorphology, Zaragoza.Google Scholar
González Trueba, J.J., Martín Moreno, R., Martónez de Pisón, E., Serrano, E., (2008). ‘Little Ice Age’ glaciation and current glaciers in the Iberian Peninsula. The Holocene 18, 4, 551–568.Google Scholar
Grimm, E.C., (1987). CONISS: a Fortran 77 program for stratigraphically constrained cluster analysis by the method of incremental sum of squares. Computers and Geosciences 13, 13–35.CrossRefGoogle Scholar
Grootes, P., Stuiver, M., White, J.W.C., Johnsen, S.J., Jouzel, J., (1993). Comparison of oxygen isotope records from the GISP2 and GRIP Greenland ice cores. Nature 366, 552–554.Google Scholar
Harding, A., Palutikof, J., Holt, T., (2009). The climate system. Woodward, J., The Physical Geography of the Mediterranean. Oxford University Press, Oxford.CrossRefGoogle Scholar
Hurrell, J.W., (1995). Decadal trends in the North Atlantic Oscillation: regional temperatures and precipitation. Science 269, 676–679.Google Scholar
Jalut, G., Dedoubat, J.J., Fontugne, M., Otto, T., (2009). Holocene circum-Mediterranean vegetation changes: climate forcing and human impact. Quaternary International 200, 4–18.Google Scholar
Jimenez-Espejo, F.J., Martínez-Ruiz, F., Rogerson, M., González-Donoso, J.M., Romero, O.E., Linares, D., Sakamoto, T., Gallego-Torres, D., Rueda Ruiz, J.L., Ortega-Huertas, M., P"rez Claros, J.A., (2008). Detrital input, productivity fluctuations, and water mass circulation in the westernmost Mediterranean Sea since the Last Glacial Maximum. Geochemistry, Geophysics, Geosystems 9, Q11U02.Google Scholar
Jiménez-Moreno, G., Fawcett, P.J., Anderson, R.S., (2008). Millennial- and centennial-scale vegetation and climate changes during the late Pleistocene and Holocene from northern New Mexico (USA). Quaternary Science Reviews 27, 1442–1452.Google Scholar
Jiménez-Moreno, G., Anderson, R.S., Atudorei, V., Toney, J.L., (2011). A high-resolution record of vegetation, climate, and fire regimes in the mixed conifer forest of northern Colorado (USA). Geological Society of America Bulletin 123, 240–254.Google Scholar
Jolly, D., Harrison, S., Damnati, B., Bonnefille, R., (1998). Simulated climate and biomes of Africa during the late Quaternary: comparison with pollen and lake status data. Quaternary Science Reviews 17, 629–657.Google Scholar
Lamb, H.F., van der Kaars, S., (1995). Vegetational response to Holocene climatic change: pollen and palaeolimnological data from the Middle Atlas, Morocco. The Holocene 5, 400–408.Google Scholar
Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A.C.M., Levrard, B., (2004). A long term numerical solution for the insolation quantities of the Earth. Astronomy and Astrophysics 428, 261–285.Google Scholar
Li, L., Bozec, A., Somot, S., Beranger, K., Bouruet-Aubertot, P., Sevault, F., Crepon, M., (2006). Regional atmospheric, marine processes and climate modeling. Lionello, P., Malanotte-Rizzoli, P., Boscolo, R., Mediterranean Climate Variability, Developments in earth and Environmental Sciences 4, Elsevier, Amsterdam. 373–397.Google Scholar
Lionello, P., Malanotte-Rizzoli, P., Boscolo, R., Alpert, P., Artale, V., Li, L., Luterbacher, J., May, W., Trigo, R., Tsimplis, M., Ulbric, U., Xoplaki, E., (2006). The Mediterranean climate: an overview of the main characteristics and issues. Lionello, P., Malanotte-Rizzoli, P., Boscolo, R., Mediterranean Climate Variability, Developments in earth and Environmental Sciences 4, Elsevier, Amsterdam. 1–26.Google Scholar
Magny, M., (2004). Holocene climate variability as reflected by mid-European lake-level fluctuations and its probable impact on prehistoric human settlements. Quaternary International 113, 65–79.Google Scholar
Magny, M., Miramont, C., Sivan, O., (2002). Assessment of the impact of climate and anthropogenic factors on Holocene Mediterranean vegetation in Europe on the basis of palaeohydrological records. Palaeogeography, Palaeoclimatology, Palaeoecology 186, 47–59.Google Scholar
Magny, M., Vannière, B., Zanchetta, G., Fouache, E., Touchais, G., Petrika, L., Coussot, C., Arnaud, F., (2009). Possible complexity of the climatic event around 4300"3800 cal. BP in the central and western Mediterranean. The Holocene 19, 823–833.Google Scholar
Martín Martín, J.M., Braga Alarcón, J.C., Gómez Pugnaire, M.T., (2010). Geological Routes of Sierra Nevada. Regional Ministry for the Environment, Junta de Andalucía.Google Scholar
Martínez-Ruiz, F.A., Paytan, M., Kastner, J.M., Gonzalez-Donoso, D., Linares, S.M., Bernasconi, , Jiménez-Espejo, F.J., (2003). A comparative study of the geochemical and mineralogical characteristics of the S1 sapropel in the western and eastern Mediterranean. Palaeogeography, Palaeoclimatology, Palaeoecology 190, 23–37.Google Scholar
Meijer, P.Th., Tuenter, E., (2007). The effect of precession-induced changes in the Mediterranean freshwater budget on circulation at shallow and intermediate depth. Journal of Marine Systems 68, 349–365.Google Scholar
Mercuri, A.M., Sadori, L., Ollero, P.U., (2011). Mediterranean and north-African cultural adaptations to mid-Holocene environmental and climatic changes. The Holocene 21, 189–206.Google Scholar
Messerli, B., (1965). Beitrage zur geomorphologie der Sierra Nevada (Andalusien). Juris Verlag, Zurich.Google Scholar
Muñoz-Díaz, D., Rodrigo, F.S., (2003). Effects of the North Atlantic Oscillation on the probability for climatic categories of local monthly rainfall in southern Spain. International Journal of Climatology 23, 381–397.Google Scholar
Obermaier, H., Carandell, J., (1916). Los glaciares cuaternarios en Sierra Nevada. Trabajos Museo Nacional Ciencias Naturales (Geología) 17, .Google Scholar
Oliva, M., (2006). Reconstrucci" paleoambiental Holocena de Sierra Nevada a partir de registres sedimentaris. Ph.D. thesis dissertation. Universitat de Barcelona, Spain.Google Scholar
Pantaleón-Cano, J., Yll, E.I., Pérez-Obiol, R., Roure, J.M., (2003). Palynological evidence for vegetational history in semi-arid areas of the western Mediterranean (Almería, Spain). The Holocene 13, 109–119.CrossRefGoogle Scholar
Pérez-Obiol, R., Jalut, G., Juliá, R., Pèlachs, A., Iriarte, M.J., Otto, T., Hernández-Beloqui, B., (2011). Mid-Holocene vegetation and climatic history of the Iberian Peninsula. The Holocene 21, 75–93.CrossRefGoogle Scholar
Pons, A., Reille, M., (1988). The Holocene- and Upper Pleistocene pollen record from Padul (Granada, Spain): a new study. Palaeogeography, Palaeoclimatology, Palaeoecology 66, 243–263.Google Scholar
Pozo-Vázquez, D., Gámiz-Ortiz, S.R., Tovar-Pescador, J., Esteban-Parra, M.J., Castro-Díez, Y., (2005). El Niño–Southern Oscillation events and associated European winter precipitation anomalies. International Journal of Climatology 25, 17–31.Google Scholar
Reed, J.M., Stevenson, A.C., Juggins, S., (2001). A multi-proxy record of Holocene climatic change in southwestern Spain: the Laguna de Medina, C"diz. The Holocene 11, 707–719.Google Scholar
Renssen, H., Brovkin, V., Fichefet, T., Goosse, H., (2003). Holocene climate instability during the termination of the African Humid Period. Geophysical Research Letters 30, 1184 http://dx.doi.org/10.1029/2002GL016636CrossRefGoogle Scholar
Roberts, N., Brayshaw, D., Kuzucuolu, C., Perez, R., Sadori, L., (2011). The mid-Holocene climatic transition in the Mediterranean: causes and consequences. The Holocene 21, 3–13.Google Scholar
Schmidt, E., (1956). Die Pflanzenwelt Spaniens. Verlag Hans Huber, Bern.Google Scholar
Schulte, L., (2002). Climatic and human influence on river systems and glacier fluctuations in southeast Spain since the Last Glacial Maximum. Quaternary International 93"94, 85–100.Google Scholar
Scott, D.B., Medioli, F.S., Schafer, C.T., (2001). Monitoring in Coastal Environments Using Foraminifera and Thecamoebian Indicators. Cambridge University Press, New York. 177.Google Scholar
Snowball, I., Sandgren, P., (2001). Application of mineral magnetic techniques to paleolimnology. Tracking Environmental Change Using Lake Sediments 2, Kluwer Academic Publishers, Dordretch. 217–237.Google Scholar
Stuiver, M., Reimer, P.J., (1993). Extended 14C database and revised CALIB radiocarbon calibration program. Radiocarbon 35, 215–230.Google Scholar
Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, F.G., v. d. Plicht, J., Spurk, M., (1998). INTCAL98 Radiocarbon age calibration 24,000"0 cal BP. Radiocarbon 40, 1041–1083.Google Scholar
Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Henderson, K.A., Brecher, H.H., Zagorodnov, V.S., Mashiotta, T.A., Lin, P.N., Mikhalenko, V.N., Hardy, D.R., Beer, J., (2002). Kilimanjaro ice core records: evidence of Holocene climate change in tropical Africa. Science 298, 589–593.Google Scholar
Tinner, W., Kaltenrieder, P., (2005). Rapid responses of high-mountain vegetation to early Holocene environmental changes in the Swiss Alps. Journal of Ecology 93, 936–947.CrossRefGoogle Scholar
Tinner, W., Theurillat, J.-P., (2003). Uppermost limit, extent, and fluctuations of the timberline and treeline ecocline in the swiss Central Alps during the past 11,500 years. Arctic, Antarctic, and Alpine Research 35, 158–169.Google Scholar
Tuenter, E., Weber, S.L., Hilgen, F.J., Lourens, L.J., (2003). The response of the African summer monsoon to remote and local forcing due to precession and obliquity. Global and Planetary Change 36, 219–235.Google Scholar
Tzedakis, P.C., (2007). Seven ambiguities in the Mediterranean palaeoenvironmental narrative. Quaternary Science Reviews 26, 2042–2066.Google Scholar
Valbuena-Carabaña, M., López de Heredia, U., Fuentes-Utrilla, P., González-Doncel, I., Gil, L., (2010). Historical and recent changes in the Spanish forests: a socio-economic process. Review of Palaeobotany and Palynology 162, 492–506.Google Scholar
Valle, F., (Ed.).(2003). Mapa de Series de Vegetación de Andalucía. Editorial Rueda, S.I., Madrid.Google Scholar
Valle, F., Gómez-Mercado, F., Mota, J.F., Díaz de la Guardia, C., (1989). Parque Natural de Cazorla. Segura y Las Villas. Guía Botánico-Ecológica, Rueda, Madrid, Spain.Google Scholar
Vanniere, B., Power, M.J., Roberts, N., Tinner, W., Carrión, J., Magny, M., Bartlein, P., Colombaroli, d., Daniau, A.L., Finsinger, W., Gil-Romera, G., Kaltenrieder, P., Pini, R., Sadori, L., Turner, R., Valsecchi, V., Vescovi, E., (2011). Circum-Mediterranean fire ativity and climate changes during the mid-Holocene environmental transition (8500"2500 cal. BP). The Holocene 21, 53–73.Google Scholar
Vernet, J.L., Faure, H., (2000). Isotopic chronology of the Sahara and the Sahel during the late Pleistocene and the early and Mid-Holocene (15 000"6000 BP). Quaternary International 68"71, 385–387.Google Scholar
Zanchetta, G., Drysdale, R.N., Hellstrom, J.C., Fallick, A.E., Isola, I., Gagan, M.K., Pareschi, M.T., (2007). Enhanced rainfall in the Western Mediterranean during deposition of sapropel S1: stalagmite evidence from Corchia cave (Central Italy). Quaternary Science Reviews 26, 279–286.Google Scholar