During the early Pliocene, two major tectonic events triggered a profound reorganization of ocean and atmospheric circulation in the eastern equatorial Pacific (EEP), in the Caribbean Sea, and on adjacent land masses: the progressive closure of the Central American Seaway (CAS) and the uplift of the Northern Andes. These affected, among other things, the mean latitudinal position of the Intertropical Convergence Zone (ITCZ). The direction of an ITCZ shift, however, is still debated, as numeric modeling results and paleoceanographic data indicate shifts in opposite directions. To provide new insights into this debate, an independent hydrological record of western equatorial South America was generated. Vegetation and climate of this area were reconstructed by pollen analysis of 46 samples from marine sediments of Ocean Drilling Program (ODP) Hole 1239A from the EEP comprising the interval between 4.7 and 4.2 Ma. The study site is sensitive to latitudinal ITCZ shifts insofar as a southward (northward) shift would result in increased (decreased) precipitation over Ecuador. The presented pollen record comprises representatives from five ecological groups: lowland rainforest, lower montane forest, upper montane forest, páramo, and broad range taxa. A broad tropical rainforest coverage persisted in the study area throughout the early Pliocene, without significant open vegetation beyond the páramo. Between 4.7 and 4.42 Ma, humidity increases, reaching its peak around 4.42 Ma and slightly decreasing again afterwards. The stable, permanently humid conditions are rather in agreement with paleoceanographic data, indicating a southward shift of the ITCZ, possibly in response to CAS closure. The presence of páramo vegetation indicates that the Ecuadorian Andes had already reached considerable elevation by the early Pliocene. Future studies could extend the hydrological record of the region further back into the late Miocene to see if a more profound atmospheric response to tectonic changes occurred earlier.
LR04 global stack of benthic
The progressive closure of the Central American Seaway (CAS) and the uplift of the Northern Andes profoundly reorganized early Pliocene ocean and atmospheric circulation in the eastern equatorial Pacific (EEP). The formation of the Isthmus of Panama, and especially the precise temporal constraints of the closure of the Panama Strait, have been the subject of numerous studies (Bartoli et al., 2005; Groeneveld et al., 2014; Hoorn and Flantua, 2015; Montes et al., 2015; Steph, 2005). A recent review based on geological, paleontological, and molecular records narrowed the formation sensu stricto down to 2.8 Ma (O'Dea et al., 2016). Temporal constraints on the restriction of the surface water flow through the gateway were established by salinity reconstructions on both sides of the isthmus (Steph et al., 2006b, Fig. 1). The salinities first start to diverge around 4.5 Ma. A major step in the seaway closure between 4.7 and 4.2 Ma was also assumed based on the comparison of mass accumulation rates of the carbonate sand fraction in the Caribbean Sea and the EEP (Haug and Tiedemann, 1998). The closure of the Central American Seaway has been associated with the development of the EEP cold tongue (EEP CT), strengthened upwelling in the EEP, the shoaling of the thermocline, and a mean latitudinal shift of the Intertropical Convergence Zone (ITCZ; Steph, 2005; Steph et al., 2006a, b, 2010). The direction of a potential shift of the ITCZ is still debated because of a discrepancy between paleoclimate reconstructions based on proxy data and numerical modeling results.
For the late Miocene, a northernmost paleoposition of the ITCZ at about
10–12
An independent record of the terrestrial hydrology for the early Pliocene from a study site that is sensitive to latitudinal ITCZ shifts could provide new insights to this debate. Schneider et al. (2014) also stress the need for reconstructions of the ITCZ in the early and mid-Pliocene in order to understand how competing effects like an ice-free Northern Hemisphere and a weak EEP CT balanced, and to reduce uncertainties of predictions. Even though changes of ocean–atmosphere linkages related to ENSO (El Niño–Southern Oscillation) and ITCZ shifts strongly impact continental precipitation in western equatorial South America, most studies so far have focused on paleoceanographic features such as sea-surface temperature (SST) and ocean stratification.
The second major tectonic process is the uplift of the Northern Andes, which
strongly altered atmospheric circulation patterns over South America. Three
major deformation phases include fan building in the lower Eocene to early
Oligocene, compression of Oligocene deposits in the Miocene and Pliocene,
and refolding during Pliocene to Recent times (Corredor, 2003).
While the uplift of the Central Andes is well investigated, only few studies
deal with the timing of uplift of the Northern Andes. Coltorti and
Ollier (2000), based on geomorphologic data, conclude that the uplift of the
Ecuadorian Andes started in the early Pliocene and continued until the
Pleistocene. More recent apatite fission track data indicate that the
Western Andean Cordillera of Ecuador was rapidly exhumed during the late
Miocene (13–9 Ma) (Spikings et al., 2005). Uplift estimates
for the Central Andes suggest that the Altiplano had reached less than half
of its modern elevation by 10 Ma, with uplift rates increasing from 0.1 mm yr
To better understand the early Pliocene vegetation and hydrology of western equatorial South America, we studied pollen and spores from the early Pliocene section (4.7–4.2 Ma) of the marine sediment record at Ocean Drilling Program (ODP) Site 1239 and compared this record to Holocene samples from the same site. In addition, we use elemental ratios to estimate variations in fluvial terrestrial input (Ríncon-Martínez et al., 2010). While other palynological studies of the region have been conducted for the mid-Pliocene to Holocene (González et al., 2006; Hooghiemstra, 1984; Seilles et al., 2016), only a few palynological records for the early Pliocene exist (Wijninga and Kuhry, 1990; Wijninga, 1996). The record contributes to elucidating how vegetation and climate in this area responded to changes in atmospheric and oceanic circulation, possibly induced by the closure of the Central American Seaway and the uplift of the Northern Andes. Therefore the main objectives of the study are, firstly, to investigate long-term vegetation and climatic changes, focusing on hydrology, in western equatorial South America and, secondly, to interpret these changes in relation to climate phenomena influencing the hydrology of the region, especially the mean latitudinal position of the ITCZ and variability related to ENSO. These objectives are approached through the following research questions: (1) what floral and vegetation changes took place in the coastal plain of western equatorial South America and the Ecuadorian Andes from 4.7 to 4.2 Ma? (2) What are the climatic implications of the vegetation change, especially in terms of hydrology? (3) What are the implications for Andean uplift, especially regarding the development of the high Andean páramo vegetation?
Modern climate (boreal summer) and vegetation and core site
positions of ODP sites 677, 846, 850, 851, 1000, 1239, and 1241; Trident core
TR163-38; and M772-056 mentioned in the text.
The climate of western equatorial South America is complex and
heterogeneous, as it is not only controlled by large-scale tropical climate
phenomena such as the ITCZ and ENSO but is also strongly influenced by
small-scale climate patterns caused by the diverse Andean topography
(Marchant et al., 2001; Niemann et al., 2010). The annual cycle of
precipitation in northwestern South America is controlled by insolation
changes. During boreal summer when insolation is strongest in the Northern
Hemisphere, the ITCZ is located at its northernmost position around
9–10
This general circulation pattern is modified by ENSO at interannual timescales. During warm El Niño events, the lowlands of Ecuador experience abundant precipitation, whereas the northwestern Ecuadorian Andes experience drought (Vuille et al., 2000). Regional climate patterns are also modified by the topography of the Andes, which pose an effective barrier for the large-scale atmospheric circulation. While precipitation patterns east of the Andes are driven by moisture-laden easterly trade winds originating over the tropical Atlantic and the Amazon Basin, the coastal areas and the western Andean slopes are dominated by air masses originating in the Pacific (Vuille et al., 2000, Fig. 2). The warm annual El Niño current which flows southward along the Colombian Pacific coast warms the air masses along the coast. This moist air brings over 6000 mm yearly precipitation to the northern coastal plain. In contrast, the coastal areas of southernmost Ecuador and northern Peru are under the influence of the Peru–Chile Current, which is a continuation of the cold Humboldt Current transporting cold and nutrient-rich waters and giving rise to a long strip of coastal desert (Balslev, 1988). The westward flow of the cold surface waters of the EEP CT to the western Pacific via the South Equatorial Current (SEC) is driven by the Walker circulation. Warm waters return eastwards via the North Equatorial Countercurrent (NECC; see Fig. 2). An abrupt transition between the cold SEC and the warm NECC is the equatorial front (EF; Pak and Zaneveld, 1974).
Ecuador is geographically divided into three main regions: the coastal plain with several rivers draining into the Pacific, the Andes, and the eastern lowlands which constitute the western margin of the Amazon Basin. The mountains form two parallel cordilleras which are separated by an inter-Andean valley. The diverse vegetation is the result of the combined effects of elevation and precipitation. In the coastal plain there is an abrupt shift from tropical lowland rainforests in the north to a desert dominated by annual xerophytic herbs in the south. This shift reflects the dependence of the vegetation on precipitation, which ranges from 100 to 6000 mm per year on the coastal plain. The western slopes of the Andes are covered by montane forest, which is partly interrupted by drier valleys in southern Ecuador (Balslev, 1988).
Along the coast, mangrove stands occur in the salt- and brackish-water tidal
zone of river estuaries and bays. They are formed by two species of
Ríncon-Martínez et al. (2010) showed that the terrigenous sediment supply at ODP Site 1239 during Pleistocene interglacials is mainly fluvial and that input of terrestrial material drops to low amounts during the drier glacial stages. Consequently, transport of pollen and spores to the ocean is also mainly fluvial (González et al., 2006). High rates of orographic precipitation characterize the western part of equatorial South America. These heavy rains quickly wash out any pollen that might be in the air and result in large discharge by the Ecuadorian rivers (Fig. 2). The Esmeraldas and Santiago rivers mainly drain the northern coastal plain of Ecuador, and the southern coastal plain is drained by several smaller rivers, which end in the Guayas River (Balslev, 1988). Moreover, the predominantly westerly winds (Fig. 2) are not favorable for eolian pollen dispersal to the ocean. Nevertheless, some transport by SE trade winds is possible and should be taken into account.
After reaching the ocean, pollen and spores might pass the Peru–Chile Trench – which is quite narrow along the Carnegie Ridge – by means of nepheloid layers at subsurface depths. Some northward transport from the Bay of Guayaquil by the Coastal Current (Fig. 2) is likely. However, the Peru–Chile Current flows too far from the coast to have strong influence on pollen and spore dispersal. We consider western Ecuador, northernmost Peru, and southwestern Colombia the main source areas of pollen and spores in sediments of ODP Site 1239.
ODP Site 1239 is located at 0
A total of 65 samples of 10 cm
For pollen identification, the Neotropical Pollen Database (Bush and Weng, 2007), a reference collection for neotropical species held at the Department of Palynology and Climate Dynamics in Göttingen, and related literature (Colinvaux et al., 1999; Hooghiemstra, 1984; Murillo and Bless, 1974, 1978; Roubik and Moreno, 1991) were used. Pollen types were grouped according to their main ecological affinity (Flantua et al., 2014; Marchant et al., 2002). The zonation of the diagrams was based on constrained incremental sum-of-squares cluster analysis (CONISS) of the pollen percentage curves, using the square root transformation method (Edwards & Cavalli–Sforza's chord distance) implemented in TILIA (Grimm, 1991; Supplement). Percentages are based on the pollen sum, which includes all pollen and fern spore types, including unidentifiable ones. Confidence intervals were calculated after Maher (1972). An initial age model for Site 1239 was established based on biostratigraphic information (Mix et al., 2003). The age model was refined by matching the benthic stable isotope records from Site 1239 with those from Site 1241 by visual identification of isotope stages (Tiedemann et al., 2007). Site 1241 has an orbitally tuned age model. Thus, this procedure resulted in an indirectly orbitally tuned age model for Site 1239, spanning the interval from 5 to 2.7 Ma (Tiedemann et al., 2007; Fig. S2). A coring gap of ca. 5 m exists between cores 35X (347 m composite depth) and 36X (352 m composite depth) of Hole 1239A (Tiedemann et al., 2007; Table AT3).
Elemental concentrations (total elemental counts) of Fe and K were measured at high resolution (every 2 cm) using an Avaatech™ X-ray fluorescence (XRF) core scanner at the Alfred Wegener Institute, Bremerhaven, with correction for dead time. Both holes A and B of ODP Site 1239 were sampled. A nondestructive measuring technique was applied, allowing rapid semi-quantitative geochemical analysis of sediment cores (Richter et al., 2006). Several studies comparing XRF core scanner data to geochemical measurements on discrete samples showed that major elements can be precisely measured with the scanner in a nondestructive way (e.g., Tjallingii et al., 2007).
Pliocene and Pleistocene palynomorph percentages (based on the total of pollen and spores) of ODP Hole 1239A for three vegetation belts, humidity indicators, grass pollen, and pollen and spore concentration per milliliter. 95 % confidence intervals as grey bars after Maher (1972). Age model for the last 5 Myr after Tiedemann et al. (2007) and for 6 to 5 Myr after Mix et al. (2003).
Five groups were established with pollen taxa grouped according to their main ecological affinity (Table 1). The groups páramo, upper montane forest, lower montane forest, and lowland rainforest represent vegetation belts with different altitudinal ranges (Hooghiemstra, 1984; Van der Hammen, 1974). To track changes of humidity, an additional group named “indicators of humid conditions” was established. This group includes those taxa that permanently need humid conditions to grow. Changes of the pollen percentages of the ecological groups for the Pliocene interval and the core top samples are shown in Figs. 3 and 5. Pollen percentages of single taxa are shown in Fig. S1.
To put the results of the detailed early Pliocene section into context of long-term changes, we plot a selection together with the results of a coarse-resolution pilot study in Fig. 3. Percentages of humidity indicators hint to slightly drier conditions at the beginning of the Pliocene. A trend towards higher palynomorph concentrations is found for the period from 6 to 2 Ma. Grass pollen percentages remain low, indicating mainly closed forest at altitudes below the páramo. Representation of lowland rainforest was low around 4.7 Ma, increased by 4.5 Ma, declined again to low levels around 3.5 Ma, and rose to remain at higher levels during the Pleistocene. Continuous presence of pollen and spores from the páramo indicates that the Ecuadorian Andes had reached high altitudes before the Pliocene.
In the early Pliocene samples, 141 different palynomorph types were
recognized, including 77 pollen and 64 fern spore types. A high percentage
of tree and shrub pollen (46 %–88 %) is present throughout the interval,
compared to low percentages of herbs and grass pollen (0 %–25 %; Fig. 4).
In most of the vegetation belts, one or two pollen or spore taxa are
overrepresented. The lowland rainforest is mainly represented by
Polypodiaceae, the lower montane forest is controlled by Cyatheaceae, and
the upper montane forest is strongly influenced by Podocarpaceae and
Palynomorph percentages of ODP Hole 1239A for the four vegetation
belts and other groups from 4.7 to 4.2 Ma. Grey shading represents the
95 % confidence intervals (after Maher, 1972). Vertical black lines
delimit the pollen zones. At the top stable oxygen isotopes of the benthic
foraminifer
List of identified pollen and spore taxa in marine ODP holes 1239A (Pliocene samples) and 1239B (core top samples; taxa in bold occurred only in core top samples) and grouping according to their main ecological affinity (Flantua et al., 2014; Marchant et al., 2002).
The pollen record of ODP Hole 1239A was divided into four main pollen zones
based on constrained cluster analysis (Fig. 4 and Fig. S1). Pollen zone I
(333.4–325.2 m b.s.f.: 4.70–4.58 Ma, 14 samples) has low pollen and spore
concentrations. It is characterized by low pollen percentages of lowland
rainforest, increases in pollen values of lower montane forest, the
percentage of fern spores, and the
Comparison of the palynomorph percentages (based on total pollen and spores) of Podocarpaceae and the different vegetation belts between two Holocene samples (black) and Pliocene samples between 4.7 and 4.2 Ma (box–whisker plots).
Two samples from the top of ODP Hole 1239B have been analyzed to facilitate
a comparison of the recent palynological signal with modern vegetation (Fig. 5 and Fig. S1). Although there is no detailed age control on these
surface/subsurface samples, a Holocene age can be assigned based on the
benthic oxygen isotope record (Rincón-Martínez et al., 2010).
Fifty-one different palynomorph types were recognized, including 29 pollen
and 22 fern spore types. The samples are characterized by low pollen and
spore concentrations of 685 and 465 grains cm
Palynomorph percentages of páramo indicators and Asteraceae
Tubuliflorae (excluding
The pollen spectrum from the páramo at ODP Site 1239 includes three
different taxa which are mainly confined to the páramo: the pollen type
The
In order to better understand the source areas and transport ways of pollen grains to the sediments, we make a comparison of the results of our two Holocene samples (Fig. S1) with that of another pollen record retrieved from the Carnegie Ridge southeast of ODP Site 1239 (TR 163-38, Fig. 2) reflecting rainfall and humidity variation of the late Pleistocene (González et al., 2006). Holocene samples of Site 1239 gave similar results showing extensive open vegetation (indicated by pollen of Poaceae, Cyperaceae, Asteraceae) and maximum relative abundance of fern spores although concentration is low (González et al., 2006). As also indicated by the elemental ratios, fluvial transport of pollen predominates in this area (González et al., 2006; Ríncon-Martínez, 2013). This is understandable, as neither ocean currents nor the wind field favor transport from Ecuador to Site 1239 (Fig. 2).
Despite the expansion of open vegetation, González et al. (2006) interpreted this record as reflecting permanently humid conditions, with disturbance processes caused by human occupation and more intense fluvial dynamics. The relatively high percentage of indicators of humid conditions in our core top samples compared to pollen zones III and IV in the early Pliocene would be in agreement with this interpretation. The core top samples from ODP Hole 1239B and the most recent part of core TR 163-38 are taken as a basis for the hydrological interpretation of the Pliocene pollen record.
The presented marine palynological record provides new information on
floristic and vegetation changes occurring along diverse ecological and
climatic gradients through the early Pliocene. The consistently high
percentage of tree and shrub pollen, compared to a low percentage of herbs
and grass pollen (
Shifts in the vegetation are driven by various parameters such as
temperature, precipitation,
Early Pliocene pollen zones I and IV show an expansion of coastal desert herbs (Amaranthaceae, Fig. S1), which coincides with low sea-surface temperatures at ODP Site 846 in the EEP, suggesting an influence of the Peru–Chile Current (continuation of the Humboldt Current) on the coastal vegetation of southern Ecuador. Remarkably, the lowland rainforest and the coastal desert herbs follow a similar trend. This seems odd at the first glance, but a possible mechanism to explain this pattern would invoke effects of El Niño, the warm phase of ENSO. The main transport agent for pollen in this region are rivers, but in the coastal desert area of southern Ecuador and northern Peru fluvial discharge rates are low (Milliman and Farnsworth, 2011). Therefore, pollen might be retained on land until an El Niño event causes severe flooding in the coastal areas (Rodbell et al., 1999) and episodically fills the rivers which transport the pollen to the ocean. Such possible effects of El Niño seem to be strongest in pollen zones I and IV where pollen percentages of not only the lowland rainforest and coastal desert herbs but also the upper montane forest fluctuate most strongly. The lowland rainforest of the coastal plain of Ecuador and western Colombia is within the present-day range of the ITCZ and expanded from 4.7 Ma onwards possibly due to a southwards displacement of the mean latitude of the ITCZ (Figs. 3 and 4).
Podocarpaceae strongly dominate the pollen spectrum in general. However, the trend in pollen percentages of Podocarpaceae diverges from that of the other pollen taxa, which may be explained by additional transport of Podocarpaceae pollen by wind. The high pollen production of Podocarpaceae and their specialized morphology (Regal, 1982) facilitate their eolian transport. In contrast, pollen from most other taxa is predominantly fluvially transported (González et al., 2006), therefore exhibiting a different pattern where high pollen concentrations correspond to high fluvial discharge in the source area. Eolian transport of Podocarpaceae explains the high pollen concentrations in pollen zone III, which occur despite less humid conditions compared to pollen zones II and IV. The increased eolian transport at 4.63 Ma and between 4.4 and 4.25 Ma is proposed here to be the result of an intensification of the easterly trade winds. Increase in trade wind strength at 4.4 Ma would be in line with a shift in the locus of maximum opal accumulation rates in the ocean associated with a shift in nutrient availability from ODP Site 850 to ODP Site 846 nearer to the continent (positions shown in Fig. 2) (Farrell et al., 1995). Dynamic modeling indicates that stronger easterlies would cause shoaling of the EEP thermocline (Zhang et al., 2012), which took place between 4.8 and 4.0 Ma (Fig. 7; Steph et al., 2006a). Related to this process, a critical step of easterly trade wind intensification, indicated by increased eolian transport of Podocarpaceae pollen, occurred between 4.4 and 4.25 Ma.
Percentages of indicators of humid conditions (ODP Site 1239, this
study),
Comparing the pollen percentages of páramo and upper montane forest, Fig. 8 indicates that UMF maxima coincide with páramo minima and SST maxima at ODP Site 846 (Lawrence et al., 2006). This might be explained by a shift of the upper montane forest to higher altitudes at the cost of the area occupied by páramo vegetation as a result of higher atmospheric temperatures and/or increased orographic precipitation in the Western Andean Cordillera caused by higher sea-surface temperatures and increased evaporation.
In order to use the existence of páramo vegetation as an indicator for
Andean elevation, the altitudinal restriction of the páramo taxa to
environments above the forest line is a prerequisite. Although no taxa
restricted to páramo were identified in the marine samples, or rather
they could not be identified due to the lack of genus-level morphological
distinction (especially
Pollen percentages of upper montane forest and páramo, and
U
The pollen record shows a continuous existence of páramo vegetation. During the warm Pliocene, the upper montane forest is assumed to have extended to altitudes similar to or even higher than those of today. Despite this upward expansion of the upper montane forest, the páramo was still present, which implies that the Western Cordillera of the Ecuadorian Andes had already gone through substantial uplift by that time. Furthermore, the pollen record has a large montane signature, which would not be the case if the Andes had reached less than half of their modern height by the early Pliocene (Coltorti and Ollier, 2000). The upper montane forest, which constitutes up to 60 % of the pollen sum, shows that montane habitats with the corresponding altitudinal belts were already existent. These findings suggest an earlier development of the high Andean páramo ecosystem than previously inferred from palynological studies of the eastern Cordillera in Colombia (Hooghiemstra et al., 2006; Van der Hammen et al., 1973). This might also be an indication that the uplift history of the Western Cordillera of Ecuador is temporally more closely related to the uplift of the Central Andes where a major phase of uplift occurred between 10 and 6 Ma (Garzione et al., 2008). In another recent palynological study, the arrival of palynomorphs from the páramo in sediments of the Amazon Fan has been documented since 5.4 Ma (Hoorn et al., 2017). Since the Amazon has its westernmost source in Peru, this signal might be related to the uplift of the Central Andes. These new records agree with paleoclimatic studies showing that modern type precipitation patterns have likely been in place since the middle Miocene (Barnes et al., 2012; Hoorn et al., 2010; Kaandorp et al., 2006), which would have required a significant orographic barrier. High Andean mountains acting as a climate divide might thus go as far back as the mid-Miocene. However, earliest evidence for páramo vegetation now points to the late Miocene.
Several studies have suggested the existence of a “permanent El Niño” during the Pliocene (e.g., Fedorov et al., 2006; Wara et al., 2005). El Niño events are characterized by a shift in the Walker circulation, resulting in exceptionally heavy precipitation particularly over the lowlands of central and southern Ecuador (Bendix and Bendix, 2006) and simultaneous below-average rainfall over the northwestern slopes of the Andes (Vuille et al., 2000). A permanent El Niño-like climate state during the early Pliocene would thus have involved permanently humid conditions with high rates of precipitation and fluvial discharge in the lowlands. Such a climate would have favored the persistence of a broad rain forest coverage and precluded the development of the desert that exists in coastal southern Ecuador today. The presented pollen record indeed indicates very humid conditions, and the only indicator of dry vegetation is a small percentage of Amaranthaceae pollen. The predicted pattern of expansion of lowland rainforest at the cost of Andean forest during permanent El Niño is not reflected in the pollen record.
The hypothesis of a permanent El Niño climate state involving a reduced
zonal Pacific sea-surface temperature gradient has recently been questioned
as sea-surface temperature reconstructions differ substantially depending on
the method. Zhang et al. (2014) claim that a zonal
temperature gradient of ca. 3
Numerical models suggesting a northward shift of the ITCZ in response to the closure of the Central American Seaway or the uplift of the Northern Andes do not necessarily disagree with an early Pliocene southward shift inferred from proxy data. Both events occurred gradually over several millions of years, and, despite recent advances in constraining these events, the timing of major phases in the uplift histories is still debated. In the case of the Central American Seaway, the timing of surface water restriction based on diverging salinities in the Caribbean and Pacific Ocean is well constrained, and numerous global oceanographic changes have been associated with it. Possibly these oceanic reorganizations did not directly trigger modifications of the atmospheric circulation (Kaandorp et al., 2006; Hoorn et al., 2010), but critical periods of uplift influencing atmospheric circulation might have occurred earlier. On the other hand, the respective model sensitivity experiments generally only consider isolated changes in single boundary conditions (e.g., closed or open Central American Seaway). Therefore, the effect of those (i.e., a northward shift of the ITCZ) might counteract the general trend of a southward shift since the late Miocene due to a decrease in the hemispheric temperature gradient (e.g., Pettke et al., 2002). Additionally, global coupled models exhibit uncertainties in the representation of ocean–atmosphere feedback and cloud–radiation feedbacks, which are especially strong in the study region (i.e., showing a double ITCZ and an extensive EEP cold tongue; Li and Xie, 2014). This is problematic also in light of the high sensitivity of the ITCZ position to slight shifts in the atmospheric energy balance (Schneider et al., 2014). Another aspect to consider is that, whereas proxy records record the transient response of the climate system over a limited period of time, the mentioned model simulations rather follow the overall equilibrium response than reproducing a stepwise process of environmental changes.
Concerning the uplift of the Northern Andes, there is still a large uncertainty about the time when the Cordilleras reached their current elevation. Moreover, phases of major uplift might have strongly differed regionally. Paleobotanists (e.g., Hooghiemstra et al., 2006; Hoorn et al., 2010; Van der Hammen et al., 1973) and some tectonic geologists (e.g., Mora et al., 2008) argue for a rapid rise of the Eastern Cordillera since 4–6 Ma, while others conclude that this is rather unlikely, implying an earlier uplift based on biomarker-based paleotemperatures (e.g., Anderson et al., 2015; Mora-Páez et al., 2016). Possibly the Pliocene oceanic reorganizations did not directly trigger modifications of the atmospheric circulation, which probably was more or less in place (Kaandorp et al., 2006; Hoorn et al., 2010). Critical periods of uplift influencing atmospheric circulation might have occurred earlier (see also above). The estimates for uplift of the Western Cordillera in Ecuador differ even more strongly and range from rapid exhumation around 13 and 9 Ma based on thermochronology (Spikings et al., 2005) to a recent uplift during the Pliocene and Pleistocene (Coltorti and Ollier, 2000). Our pollen record from the páramo shows that the Ecuadorian Andes must have already reached close to modern elevations by the early Pliocene in line with inferences of Hoorn et al. (2017) and Bermúdez et al. (2015). If an early Andean uplift is assumed, the atmospheric response predicted by the model would have occurred earlier, which would also be in agreement with proxy data indicating a northern position of the ITCZ during the late Miocene (Hovan, 1995).
Overall, even if the timing and identification of major steps in the shoaling and restriction of the Central American Seaway or in the uplift of the Northern Andes are resolved, the critical threshold for profound changes in atmospheric circulation and climate may have occurred at any time during the tectonic processes. Within the analyzed time window, large changes in atmospheric circulation which have been proposed as a response to the closure of the Central American Seaway (Ravelo et al., 2004) are absent.
Between 4.7 and 4.2 Ma, a permanently humid climate with broad rainforest coverage existed in western equatorial South America. No evidence was found for a permanent El Niño-like climate state, but strong fluctuations in the vegetation between 4.7 and 4.55 Ma and between 4.26 and 4.2 Ma indicate strong periodic El Niño variability at this time. Hydrological changes between 4.55 and 4.26 Ma are attributed to gradual shifts of the Intertropical Convergence Zone, which reached its southernmost position around 4.42 Ma and shifted slightly north afterwards.
The most prominent shift recorded during the early Pliocene is an increase in the representation of the lowland rainforest around 4.5 Ma.
Between 4.41 and 4.26 Ma, an increased eolian influx of Podocarpaceae pollen indicates an increased strength of the easterly trade winds, which is presumably related to the shoaling of the EEP thermocline.
Results from proxy data and numerical modeling studies regarding the position of the ITCZ during the early Pliocene are not necessarily contradictory. Considering the temporal uncertainties regarding major steps of CAS closure and uplift of the Northern Andes, the proposed northward shift of the ITCZ in response to these events might have occurred much earlier (e.g., during the middle to late Miocene).
The continuous presence of páramo vegetation since 6 Ma implies that the Ecuadorian Andes had already reached an elevation suitable for the development of vegetation above the upper forest line by the latest Miocene. We present new paleobotanical evidence indicating an earlier development of páramo vegetation than previously suggested by terrestrial paleobotanical records.
The underlying research data are stored in PANGAEA as datasets
PANGAEA.884280, PANGAEA.891294 and PANGAEA.884153, which are combined in
PANGAEA.884285 (
The supplement related to this article is available online at:
LD and FG conceived the idea; LD, FG, and FL carried out the analyses. FG prepared the manuscript with contributions from all co-authors.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Global Challenges for our Common Future: a paleoscience perspective” – PAGES Young Scientists Meeting 2017. It is a result of the 3rd Young Scientists Meeting (YSM), Morillo de Tou, Spain, 7–9 May 2017.
We thank Carina Hoorn, Jun Tian, Henry Hooghiemstra, Suzette Flantua and an anonymous referee for their insightful comments on the original manuscript. This project was funded by the Deutsche Forschungsgemeinschaft (DFG) through the TROPSAP project (DU221/6) and via the DFG Research Center/Cluster of Excellence “The Ocean in the Earth System – MARUM”. The first author thanks GLOMAR – Bremen International Graduate School for Marine Sciences, University of Bremen, Germany, for support. We acknowledge the IODP Gulf Coast Repository (GCR) for their assistance in providing the core samples. The article processing charges for this open-access publication were covered by the University of Bremen. Edited by: Yancheng Zhang Reviewed by: Carina Hoorn, Jun Tian, and one anonymous referee