PALEOCEANOGRAPHY

Middle Miocene closure of the CentralAmerican SeawayC. Montes,1* A. Cardona,2 C. Jaramillo,3 A. Pardo,4 J. C. Silva,5 V. Valencia,6 C. Ayala,7

L. C. Pérez-Angel,1 L. A. Rodriguez-Parra,1 V. Ramirez,8 H. Niño8

Uranium-lead geochronology in detrital zircons and provenance analyses in eight boreholesand two surface stratigraphic sections in the northern Andes provide insight into thetime of closure of the Central American Seaway. The timing of this closure has beencorrelated with Plio-Pleistocene global oceanographic, atmospheric, and biotic events.We found that a uniquely Panamanian Eocene detrital zircon fingerprint is pronounced inmiddle Miocene fluvial and shallow marine strata cropping out in the northern Andesbut is absent in underlying lower Miocene and Oligocene strata. We contend that thisfingerprint demonstrates a fluvial connection, and therefore the absence of an interveningseaway, between the Panama arc and South America in middle Miocene times; the CentralAmerican Seaway had vanished by that time.

Closure of the Central American Seaway,defined here as the deep oceanic seawayalong the tectonic boundary of the SouthAmerican plate and the Panama arc, isthought to have modified the salinity of the

Caribbean Sea, ultimately affecting ocean circu-lation patterns and global climate (1), as well asto have triggered the Great American Biotic In-terchange (2). However, the role of the formation

of the Panamanian Isthmus in such global changesremains controversial, in part because of the dif-ficulty of establishing a precise chronology ofseaway closure (3). Data on the chronology ofIsthmus emergence suggests that the closure notonly occurred earlier than previously thought (4)but also may have resulted from factors otherthan the emergence of currently high terrain inPanama (5, 6).

The Uramita suture (7) separates the youngPanama arc to the west from the old Andeanterranes to the east (Fig. 1). These are mutuallyexclusive geochronological domains that are ide-ally suited for documenting the time of detritalexchange. The young Panama magmatic arc wasbuilt on an oceanic plateau substrate (8) duringlatest Cretaceous to Eocene times [67 to 39 mil-lion years ago (Ma), with a peak around 50 Ma](6, 9), with renewed magmatic activity as youngas 19 Ma (10) east of the Canal Basin and 10 Maand younger west of it (11). To further character-ize the Panama magmatic arc fingerprint, wedated a string of incompletely mapped graniticplutons along the northeastern coast of Panamaand western Colombia (Fig. 1C), obtaining eightU/Pb magmatic zircon ages ranging between59 and 42 Ma (217 U/Pb analyses; table S1). Thenorthern Andes, in contrast, include magmaticrocks accreted during latest Cretaceous times(8) to a core of plutonometamorphic rock oflate Precambrian (12–14) and Permo-Triassic age(14, 15), and plutonic rocks of Jurassic to Creta-ceous age (16). Middle Eocene magmatism is

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Fig. 1.Tectonic setting of the study area and loca-tion of samples. See Table 1. Thick lines representmajor boundaries (28). Zero-milligal (0 mgal) contour(29) highlights the geodynamic continuity of thePanama arc; there are no structural breaks betweenthe Uramita suture and the Canal Basin. (A and B)Detrital zircon ages recovered from (A) lower Miocenestrata in the Canal Basin (6) and (B) Oligocene-Miocenestrata in the Nuevo Mundo Syncline (18) and riversdraining the Eastern and Central cordilleras (19). (C)New U/Pb zircon ages for granitoids of the Panamaarc; data point error ellipses are 68.3% confidence(see table S1).

1Universidad de los Andes, Bogotá, Colombia. 2UniversidadNacional de Colombia, Medellín, Colombia. 3SmithsonianTropical Research Institute, Ciudad de Panamá, Panamá.4Universidad de Caldas, Manizales, Colombia. 5University ofHouston, Houston, TX 77004, USA. 6Washington StateUniversity, Pullman, WA 99164, USA. 7Corporación GeológicaAres, Bogotá, Colombia. 8Ecopetrol, Bogotá, Colombia.*Corresponding author. E-mail: cmontes@uniandes.edu.co

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absent in the northern Andes (17–19). Therefore,detrital zircons of Eocene age can be used totrack the detrital contribution of the Panama arcto sedimentary basins of northwestern SouthAmerica.To track detrital contributions from the Panama

arc, we sampled fluvial strata at the westernflank of the Central Cordillera of Colombia (siteSA in Fig. 1), following a roughly northeasterntrend toward shallow marine strata (sites SMPand BH1 to BH8) in the LowerMagdalena Basin.Seven stratigraphic levels of middle Miocene ageand 11 stratigraphic levels of Oligocene to earlyMiocene age were sampled in eight boreholesand two surface stratigraphic sections (Table 1).We obtained 18 U/Pb detrital zircon ages, as wellas petrographic and heavy mineral analyses ofthe sedimentary rock (1654U/Pb analyses; tablesS2 to S5). All detrital zircon samples recoveredfrom Oligocene and Miocene strata containedtypical north Andean detrital signatures that in-cluded late Precambrian, Permo-Triassic, andLate Cretaceous populations. Middle Miocenestrata, however, contained an additional Eocenemagmatic zircon population that was absent inolder strata (Fig. 2).To establish the age of strata sampled in the

Lower Magdalena Basin, we used published fo-raminiferal and palynological studies performedon the same boreholes where we sampled, fur-

ther bracketed by our detrital zircon minimalages (Fig. 2). In the western flank of the CentralCordillera, we relied on mapped cross-cuttingrelationships (20) of volcanic and subvolcanicrocks interbedded with and intruding fluvial,coal-bearing strata. Available geochronology (21)and palynological (22) studies along with our de-trital zircon minimal ages were used to establishages of the strata sampled (fig. S3).Because the geochronological makeup of the

northern Andes is incompletely known, we usedpublishedU/Pb detrital zircon data as proxies forthe magmatic age distribution of the easternPanama arc and the northern Andes. We usedthe Oligocene-Miocene strata of the Canal Basinas a proxy for the Panama arc (Fig. 1A) (6). Forthe northern Andes (Fig. 1B), we used two datasets as proxies of its geochronological makeup:Oligocene-Miocene strata of the Nuevo MundoSyncline (18) and active-sediment river samplesdraining the Eastern and Central Cordilleras (19).We found that detrital zircons from basins andrivers in the northern Andes are decidedly olderthan those from Panama; the mean for NuevoMundo Syncline and active-sediment river sam-ples is 304.1Ma, whereas themean for Panama is49.7Ma (Kolmogorov-Smirnov test,P<0.001,D=0.98; fig. S2, A and B). Nuevo Mundo Synclineand active-sediment river samples have an agerange of 51.2 to 2675.4 Ma, with only 16 ages be-

tween 51.2 and 63.1 Ma (mean 56 Ma). Panamaages range from 17.6 to 65.1 Ma (fig. S2B).Oligocene to middle Miocene strata sampled

in the northern Andes can be separated into twoage groups according to their detrital zircon pop-ulations: one containing an Eocene populationand another missing it (Fig. 2). The Oligocene–early Miocene strata show an age range of 54Mato 3103.6 Ma. Only two of 1045 zircons have agesyounger than 65.1 Ma (54 and 64 Ma). In con-trast, middle Miocene sandy strata in the samesampling sites (Fig. 2A) show an age range from13.1 Ma to 3189.9 Ma. A large number of them(103 of 609) have ages younger than 65.1 Ma,with a mean age of 36.8 Ma, slightly youngerthan the mean age of Panamanian detrital zir-cons (mean Panama = 49.7 Ma, Kolmogorov-Smirnov test, P < 0.001, D = 0.53; fig. S2).The Eocene detrital zircon population docu-

mented in middle Miocene strata of northwest-ern South America (Fig. 2A) could have only beenderived from the emerged Panama arc, as thereare no igneous bodies of that age in the northernAndes (18, 19). Themagmatic roots of the Panamaarc had been cooling (5, 6), emerging, and erod-ing (23) since at least late Eocene times (6);therefore, they were available as source areas bymiddleMiocene times. Both fluvial coal-bearingstrata (20, 24) and shallow marine strata ofmiddle Miocene age contain the Panama arc

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Table 1. Sample summary. See fig. S1 for stratigraphic location of samples.

Sample/localityname

Detrital zirconsample name

Latitude(decimaldegrees)

Longitude(decimaldegrees)

Sample type Age

U/Pb in detrital zircons from boreholesBH1 SI-5796 9.4699 –75.0015 Cored borehole Middle Miocene

SI-6816 9.4699 –75.0015 Cored borehole Oligocene–early MioceneBH2 SI-6635 9.3651 –74.4474 Drill cuttings Oligocene–early MioceneBH3 SI-5900 9.8907 –74.6344 Drill cuttings Middle MioceneBH4 SI-6878 9.7160 –75.1124 Cored borehole Middle MioceneBH5 SI-6703 8.6716 –74.5976 Cored borehole Oligocene–early MioceneBH6 SI-6657 8.1702 –75.4761 Drill cuttings Middle Miocene

SI-6672 8.1702 –75.4761 Drill cuttings Oligocene–early MioceneBH7 SI-7045 8.5168 –76.1256 Cored borehole Middle MioceneBH8 SI-6833 8.2680 –76.2033 Drill cuttings Oligocene–early Miocene

U/Pb in detrital zircons from surface stratigraphic sectionsSMP SI-6997 8.5488 –75.6922 Hand sample Oligocene–early MioceneSA 030337-2 6.4513 –75.7403 Hand sample Oligocene–early Miocene

030337-6 6.4513 –75.7403 Hand sample Oligocene–early Miocene030339 6.5414 –75.7754 Hand sample Oligocene–early Miocene030357 6.4085 –75.7241 Hand sample Middle Miocene030359 6.4166 –75.7255 Hand sample Middle Miocene030362 6.5064 –75.8194 Hand sample Oligocene–early Miocene

U/Pb in magmatic zircons from granitoid surface samplesDV-165 5.7680 –76.2490 Hand sample 43.8 T 0.8 MaDV-167 5.7709 –76.2475 Hand sample 42.5 T 1.3 MaGA-001 8.5094 –77.3371 Hand sample 49.5 T 0.9 MaVM-001 8.4993 –77.3221 Hand sample 49.7 T 1.6 MaVM-003 8.4847 –77.3402 Hand sample 49.5 T 1.1 MaSI-860 8.8370 –77.6218 Hand sample 59.0 T 1.9 MaSI-900 8.6638 –77.4321 Hand sample 58.6 T 1.6 MaSI-1092 9.2985 –78.1047 Hand sample 58.3 T 1.0 Ma

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signature (Fig. 2). This signature—in fluvialstrata to the south and in shallow marine stratato the north—suggests that the Panama arc had

docked and emerged and was shedding detritalmaterial to north-bound currents parallel to theUramita Suture, similar to today’s Cauca River

(Fig. 1), and to northeast-bound coastal currents(Fig. 3).Our results imply that by middle Miocene

times (13 to 15 Ma), rivers originating in thePanama arc were transporting sediment to theshallow marine basins of northern South Amer-ica. This implies that (i) at least a segment of thePanama arc, including an emerged (6) Mandebatholith and San Blas Range (Figs. 1 and 3), hadalready docked and (ii) the Central AmericanSeaway was closed. Continued Caribbean-Pacificwater exchange may have taken place along nar-row, shallow, and transient channels that frag-mented (5) the Isthmus west of the Canal Basin(4) (Fig. 3). These results support recent paleo-ceanographic studies (25, 26) that showadecreasein the transport of deep and intermediate Pacificwaters into the Caribbean by 10 to 11 Ma, prob-ably related to a closingCentral American Seaway.Recorded changes in Caribbean water salinity at~4.2 Ma (1), and a delay of nearly 10 Ma in theGreat American Biotic Interchange (2) after thefirst detrital loads crossed the Isthmus, could beunrelated to seaway closure and instead may belinked to Plio-Pleistocene global climatic tran-sitions (3, 27).

REFERENCES AND NOTES

1. G. H. Haug, R. Tiedemann, R. Zahn, A. C. Ravelo, Geology 29,207–210 (2001).

2. L. G. Marshall, S. D. Webb, J. J. Sepkoski Jr., D. M. Raup,Science 215, 1351–1357 (1982).

3. P. Molnar, Paleoceanography 23, 15 (2008).4. A. G. Coates, L. S. Collins, M.-P. Aubry, W. A. Berggren,

Geol. Soc. Am. Bull. 116, 1327–1344 (2004).5. D. W. Farris et al., Geology 39, 1007–1010 (2011).6. C. Montes et al., Geol. Soc. Am. Bull. 124, 780–799

(2012).7. H. Duque-Caro, Palaeogeogr. Palaeoclimatol. Palaeoecol. 77,

203–234 (1990).8. A. C. Kerr, J. Tarney, Geology 33, 269–272 (2005).9. W. Wegner, G. Wörner, M. E. Harmon, B. R. Jicha, Geol. Soc.

Am. Bull. 123, 703–724 (2011).10. S. Whattam et al., Lithos 142–143, 226–244 (2012).11. P. Hidalgo, T. Vogel, T. Rooney, R. Currier, P. Layer, Contrib.

Mineral. Petrol. 162, 1115–1138 (2011).12. P. A. Restrepo-Pace, J. Ruiz, G. Gehrels, M. Cosca, Earth

Planet. Sci. Lett. 150, 427–441 (1997).13. U. G. Cordani, A. Cardona, D. M. Jimenez, D. Liu, A. P. Nutman,

Geol. Soc. London Spec. Publ. 246, 329–346 (2005).14. C. J. Vinasco, U. G. Cordani, H. Gonzalez, M. Weber, C. Pelaez,

J. S. Am. Earth Sci. 21, 355–371 (2006).15. A. Cardona et al., J. S. Am. Earth Sci. 29, 772–783 (2010).16. J. A. Aspden, W. J. McCourt, M. Brook, J. Geol. Soc. London

144, 893–905 (1987).17. G. Bayona et al., Earth Planet. Sci. Lett. 331–332, 97–111

(2012).18. J. Nie et al., Geology 38, 451–454 (2010).19. J. Nie et al., Earth Sci. Rev. 110, 111–126 (2012).20. E. Grosse, Estudio Geológico del Terciario Carbonífero de

Antioquia en la parte occidental de la Cordillera Central deColombia entre el rio Arma y Sacaojal [Dietrich Reimer(Ernst Vohsen), Berlin, 1926].

21. G. Rodriguez, G. Zapata, Bol. Geol. 36, 85–102 (2014).22. T. Van der Hammen, Bol. Geol. 6, 1–56 (1958).23. W. P. Woodring, U.S. Geol. Surv. Prof. Pap. 306-A, 145

(1957).24. J. Silva Tamayo, G. Sierra, L. Correa, J. S. Am. Earth Sci. 26,

369–382 (2008).25. A. H. Osborne et al., Paleoceanography 29, 715–729 (2014).26. P. Sepulchre et al., Paleoceanography 29, 176–189 (2014).27. A. M. Mestas Nuñez, P. Molnar, Paleoceanography 29, 508–517

(2014).28. C. Montes et al., J. Geophys. Res. 117, B04105 (2012).29. G. K. Westbrook, in The Geology of North America, G. Dengo,

J. E. Case, Eds. (Geological Society of America, Boulder,CO, 1990), vol. H, Plate 7.

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Fig. 2. Detrital U/Pb zirconpopulations for all the samplesrecovered. (A) Middle Miocenestrata; (B) Lower Miocene andOligocene strata of the northernAndes. Data are binned insections of 10 million years(see tables S2 and S3).The youngest detrital zirconis labeled on top of the leftmostbin. See Table 1 and Fig. 1 forsample location.

Fig. 3. Paleogeographic recon-struction (28) of the Panama arcand northwestern South Americaduring middle Miocene times (13to 15 Ma).The first detrital loadsfrom Panama arrived in one of two,or both, paths to the basins ofnorthwestern South America: (i)along coastal currents transportingdetritus product of the erosion ofexposed plutonic rocks along thenorthern coast of the Panama arcand/or (ii) along fluvial channelsdraining emerging ranges parallelto the length of the Isthmus. TheEl Valle volcano, an edifice risingfrom sea level starting before 10 Ma(11), would only allow shallow andtransient seaways between 15 and10 Ma, as the Canal basin wasconnected to North America by aland bridge from Oligocene untilmiddle Miocene times (30).

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30. M. X. Kirby, B. MacFadden, Palaeogeogr. Palaeoclimatol.Palaeoecol. 228, 193–202 (2005).

ACKNOWLEDGMENTS

Supported by Ecopetrol-ICP “Cronología de la Deformación en lasCuencas Subandinas,” Smithsonian Institution, Uniandes P12.160422.002/001, Autoridad del Canal de Panama (ACP), the MarkTupper Fellowship, Ricardo Perez S.A.; NSF grant EAR 0824299 and

OISE, EAR, DRL 0966884, Colciencias, and the National GeographicSociety. We thank N. Hoyos, D. Villagomez, A. O’Dea, C. Bustamante,O. Montenegro, and C. Ojeda. All the data reported in this manuscriptare presented in the main paper and in the supplementary materials.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/348/6231/226/suppl/DC1Materials and Methods

Supplementary TextFigs. S1 to S3Tables S1 to S5References (31–42)

12 November 2014; accepted 2 March 201510.1126/science.aaa2815

EARTH HISTORY

Ocean acidification and thePermo-Triassic mass extinctionM. O. Clarkson,1*† S. A. Kasemann,2 R. A. Wood,1 T. M. Lenton,3 S. J. Daines,3

S. Richoz,4 F. Ohnemueller,2 A. Meixner,2 S. W. Poulton,5 E. T. Tipper6

Ocean acidification triggered by Siberian Trap volcanism was a possible kill mechanismfor the Permo-Triassic Boundary mass extinction, but direct evidence for an acidificationevent is lacking. We present a high-resolution seawater pH record across this interval,using boron isotope data combined with a quantitative modeling approach. In the latestPermian, increased ocean alkalinity primed the Earth system with a low level ofatmospheric CO2 and a high ocean buffering capacity. The first phase of extinctionwas coincident with a slow injection of carbon into the atmosphere, and oceanpH remained stable. During the second extinction pulse, however, a rapid and largeinjection of carbon caused an abrupt acidification event that drove the preferential lossof heavily calcified marine biota.

The Permo-Triassic Boundary (PTB) massextinction, at ~252 million years ago (Ma),represents the most catastrophic loss ofbiodiversity in geological history and playeda major role in dictating the subsequent

evolution of modern ecosystems (1). The PTB ex-tinction event spanned ~60,000 years (2) andcan be resolved into two distinct marine extinc-tion pulses (3). The first occurred in the latestPermian [Extinction Pulse 1 (EP1)] and was fol-lowed by an interval of temporary recovery be-fore the second pulse (EP2), which occurred inthe earliest Triassic. The direct cause of the massextinction is widely debated, with a diverse rangeof overlapping mechanisms proposed, includingwidespread water column anoxia (4), euxinia (5),global warming (6), and ocean acidification (7).Models of PTB ocean acidification suggest that

a massive and rapid release of CO2 from SiberianTrap volcanism acidified the ocean (7). Indirectevidence for acidification comes from the inter-pretation of faunal turnover records (3, 8), poten-tial dissolution surfaces (9), and Ca isotope data

(7). A rapid input of carbon is also potentiallyrecorded in the negative carbon isotope excur-sion (CIE) that characterizes the PTB interval(10, 11). The interpretation of these records is,however, debated (12–16) and is of great impor-tance to understanding the current threat ofanthropogenically driven ocean acidification (11).To test the ocean acidification hypothesis, we

have constructed a proxy record of ocean pHacross the PTB using the boron isotope compo-sition of marine carbonates (d11B) (17). We thenused a carbon cycle model (supplementary text)to explore ocean carbonate chemistry and pHscenarios that are consistent with our d11B dataand published records of carbon cycle distur-bance and environmental conditions. Throughthis combinedgeochemical, geological, andmodel-ing approach, we are able to produce an envelopethat encompasses the most realistic range in pH,which then allows us to resolve three distinctchronological phases of carbon cycle perturba-tion, eachwith very different environmental con-sequences for the Late Permian–Early TriassicEarth system.We analyzed boron and carbon isotope data

from two complementary transects in a shallowmarine, open-water carbonate succession from theUnited Arab Emirates (U.A.E.), where deposi-tional facies and stable carbon isotope ratio(d13C) are well constrained (18). During thePTB interval, the U.A.E. formed an expansivecarbonate platform that remained connectedto the central Neo-Tethyan Ocean (Fig. 1A) (18).Conodont stratigraphy and the distinct d13C curveare used to constrain the age model (17).

The PTB in the Tethys is characterized by twonegative d13C excursions interrupted by a short-term positive event (10). There is no consensus asto the cause of this “rebound” event and so weinstead focus on the broader d13C trend. Our d13Ctransect (Fig. 1B) starts in the Changhsingian(Late Permian) with a gradual decreasing trend,interrupted by the first negative shift in d13C atEP1 (at 53 m, ~251.96 Ma) (Figs. 1B and 2). This isfollowed by the minor positive rebound event (at54 m, ~251.95 Ma) (Figs. 1B and 2) before theminima of the second phase of the negative CIE(58 to 60 m, ~251.92 Ma) (Figs. 1B and 2) thatmarks the PTB itself. After the CIE minimum,d13C gradually increases to ~1.8 per mil (‰) andremains relatively stable during the earliestTriassic and across EP2.Our boron isotope record shows a different

pattern to the carbon isotope excursion. Theboron isotope ratio (d11B) is persistently low(Fig. 1C) at the start of our record during thelate-Changhsingian, with an average of 10.9 T0.9‰ (1s). This is in agreement with d11B values(average of 10.6 T 0.6‰, 1s) reported for early-Permian brachiopods (19). Further up the section(at ~40m, ~252.04Ma) (Fig. 1C), there is a steppedincrease in d11B to 15.3 T 0.8‰ (propagateduncertainty, 2sf) and by implication an increasein ocean pH of ~0.4 to 0.5 (Fig. 2). d11B valuesthen remain relatively stable, scattering around14.7 T 1.0‰ (1s) and implying variations within0.1 to 0.2 pH, into the Early Griesbachian (EarlyTriassic) and hence across EP1 and the period ofcarbon cycle disturbance (Figs. 1 and 2).After the d13C increase and stabilization (at

~85 m, ~251.88 Ma) (Fig. 1), d11B begins to de-crease rapidly to 8.2 T 1.2‰ (2sf), implying asharp drop in pH of ~0.6 to 0.7. The d11B min-imum is coincident with the interval identifiedas EP2. This ocean acidification event is short-lived (~10,000 years), and d11B values quickly re-cover toward the more alkaline values evidentduring EP1 (average of ~14‰).The initial rise in ocean pH of ~0.4 to 0.5 units

during the Late Permian (Fig. 2) suggests a largeincrease in carbonate alkalinity (20). We are ableto simulate the observed rise in d11B and pHthrough different model combinations of in-creasing silicate weathering, increased pyrite dep-osition (21), an increase in carbonate weathering,and a decrease in shallow marine carbonate dep-ositional area (supplementary text). Both sili-cate weathering and pyrite deposition result in alarge drop in partial pressure of CO2 (PCO2) (andtemperature) for a given increase in pH andsaturation state (W). There is no evidence for alarge drop in PCO2, and independent proxy data

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1School of Geosciences, University of Edinburgh, West MainsRoad, Edinburgh EH9 3FE, UK. 2Faculty of Geosciencesand MARUM–Center for Marine Environmental Sciences,University of Bremen, 28334 Bremen, Germany. 3Collegeof Life and Environmental Sciences, University of Exeter,Laver Building, North Parks Road, Exeter EX4 4QE, UK.4Institute of Earth Sciences, NAWI Graz, University of Graz,Heinrichstraße 26, 8010 Graz, Austria. 5School of Earth andEnvironment, University of Leeds, Leeds LS2 9JT, UK.6Department of Earth Sciences, University of Cambridge,Downing Street, Cambridge CB2 3EQ, UK.*Corresponding author. E-mail: matthew.clarkson@otago.ac.nz†Present address: Department of Chemistry, University of Otago,Union Street, Dunedin, 9016, Post Office Box 56, New Zealand.

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Middle Miocene closure of the Central American Seaway

Ramirez and H. NiñoC. Montes, A. Cardona, C. Jaramillo, A. Pardo, J. C. Silva, V. Valencia, C. Ayala, L. C. Pérez-Angel, L. A. Rodriguez-Parra, V.

DOI: 10.1126/science.aaa2815 (6231), 226-229.348Science 

, this issue p. 226; see also p. 186Sciencethe Americas need to be reconsidered.and Pacific had ended by then. If this is true, then many models of paleo-ocean circulation and biotic exchange betweenbasins of northern South America. One interpretation of this finding is that large-scale ocean flow between the Atlantic Flantua). The presence of the minerals indicates that rivers were flowing from the Panama Arc into the shallow marineappear in South America during the Middle Miocene, 15 to 13 million years ago (see the Perspective by Hoorn and

report that certain minerals of Panamanian provenance began toet al.million years earlier than is believed. Montes The Central American Seaway, which once separated the Panama Arc from South America, may have closed 10

Early closing between oceans

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