New Holocene pollen records from the Brazilian Caatinga

We present two pollen diagrams from the semi-arid Caatinga of the Catimbau National Park, in Pernambuco and from a Mauritia palm forest in the Caatinga/Cerrado ecotone of southern Piauí, NE Brazil, spanning the last 10,000 cal. yrs BP and the last 1,750 cal yrs BP, respectively. These two records contain a signature of the local vegetation and permit the correlation of the pollen signal with regional climatic changes. The Catimbau record shows Zizyphus sp., a typical Caatinga taxon, in all three pollen zones indicating regional Caatinga vegetation and the predominance of local arboreal taxa adapted to high humidity from 10,000 to ca. 6,000 cal. yrs BP with a gradual tendency towards drier conditions revealed by a deposition hiatus between 6,000 to ca. 2,000 cal. yrs BP. This abrupt loss of sediments in both localities is interpreted as a consequence of the establishment of modern semi-arid climates. The subsequent return of humidity is signaled by increased sedimentation rates and C date inversions in agreement with high precipitation, revealed by σO ratios in speleothems from NE Brazil. Modern sediments deposited in the last 500 years reflect local conditions with the maintenance of humidity by geological faulting and surfacing water tables.


INTRODUCTION
The Brazilian Caatinga appeared for the first time in the international scientific literature with the seminal study Flora Brasiliensis by Karl Friedrich von Martius in 1817, followed by various contributions on the physical and floristic characteristics of this semi-arid tropical ecosystem (Andrade-Lima 1953, 1977, Rizzini 1963, Rizzini and Mattos Filho 1992, Barbosa et al. 2006).Until recently, the Late Quaternary history of this vegetation had not been available due to a lack of palynological studies, which require organic-rich and unoxidized VANDA B. DE MEDEIROS et al. sediments.These were later found in the Icatu River Valley, in the mid-São Francisco River, state of Bahia, by De Oliveira et al. (1999) and revealing continuous sedimentation in the last 11,000 years.The authors observed the predominance of rainforest taxa during the Pleistocene/Holocene transition, indicating a humid phase in the Early Holocene, with a gradual loss of humidity.According to the Icatu pollen record, it became clear that rainforest as well as palm forest (buritizal) decreased gradually in abundance in that valley until the mid-Holocene and was followed by the establishment of semi-arid climate at ca. 4,200 yrs BP.
Among other studies that contribute to the understanding of Holocene climates within the Caatinga is that of Behling et al. (2000), who analyzed the pollen signal in marine sediments deposited at the Jaguaribe River delta, in the continental platform, 90 km off the coast of Ceará.These authors show that the terrestrial signal started to decrease after 8,500 yrs BP, thus suggesting the establishment of semi-arid conditions.However, it is noteworthy the return of humid climates after ca.3,200 yrs BP in high elevation regions of the semi-arid Caatinga domain of Ceará, Piauí and Paraíba (Pessenda et al. 2010) and in Serra do Maranguape, Ceará (Montade et al. 2014).In fact, the modern vegetation pattern within the Caatinga is predominantly xerophytic in the lowlands in contrast to the conspicuous presence of tropical forests containing Amazonian and Atlantic taxa in elevations higher than 1,000 meters, known as "Brejos de altitude", maintained by orographic rains which create unusual high rainfall patterns under lower temperatures within the semi-arid domain (Sales et al. 1998).
Other paleoclimatic analyses within the Caatinga region, derived from geochemical and geological data, support unstable climates during the Holocene, with a wetter early phase, followed by dryer conditions at ca. 4,000 yrs BP, intercalated with a strongly rainy period ca.2,000 yrs BP (Cruz et al. 2009, Novello et al. 2012, Nace et al. 2014, Mendes 2016).
Despite the recent advances in our knowledge of the vegetation and climatic dynamics, the Caatinga ecosystem remains the least known in the literature in terms of its vegetational and climate evolution.In this present contribution, we provide two new Holocene pollen records from the semi-arid region of central Pernambuco and southern Piauí, both located in an area with the lowest precipitation levels in this ecosystem, known in Brazil as Sertão or Polígono da Seca (Drought Polygon), in order to contribute to the understanding of the vegetational, climate and ecological processes during the Holocene.

STUDY SITES
Pollen analyses were conducted in two localities in the Caatinga domain: Vale do Catimbau, Pernambuco and Bom Jesus, Piaui, shown on Figure 1.
In the first, sediments were collected from a peatbog 0.5 km long x 0.12 km wide in the Vale do Catimbau National Park, Arcoverde municipality, Pernambuco (8º29'26"S; 37º11'20"W), at 740 m elevation.The climate is semi-arid, with a welldefined wet season between February and August, mean annual precipitation of 700 mm, and mean annual temperature of 22.9 ºC.The geological setting of the Vale do Catimbau region is controlled by the Jatobá Basin, of Silurian-Devonian age (CPRM 1964), containing the Tacaratu Formation, composed of sandstones and conglomerates.The sedimentary rocks establish an important deep aquifer in a large section of northeastern Brazil.Geological faults in the Catimbau region (Gomes 1995), especially the São José Mountain range are responsible for surfacing waters conducive to peatbog formation (Nascimento 2008).The modern vegetation cover in Vale do Catimbau includes typical Caatinga genera such as the arboreal and herbaceous Astronium, Cassia, Ceiba, Cereus, Croton, Jatropha, Manihot and Tabebuia.Nearby islands of Cerrado contain the Krameria, Hymenaea, Senna, Guapira and Tocoyena and in some humid areas, rainforest elements such as Mauritia, Sauvagesia and Justicia are common.In some areas Orbignya (Attalea) palms (babaçu) is predominant (Sales et al. 1998), especially around the peatbog sampled for the present pollen study.
The second site, at Bom Jesus, Piaui, is located at a Caatinga/Cerrado ecotone, thus reflecting a floristic mosaic composed of species belonging to those ecosystems.Organic-rich sediments, forming a superficial peatbog underlain by mineral sediments, were collected in a location named Veredas, (9 º 13'40.59"Sand 44 º 28'0.92"W),part of an extensive Mauritia flexuosa palm forest.The sampling location is located within an ephemeral drainage system under a semi-arid climate with a wet season from October to April, mean annual precipitation of 1,100 mm and mean annual temperature of 26.5ºC, at 270 m elevation (EMBRAPA 2016).The Bom Jesus Mauritia flexuosa palm forests (buritizais) are located in humid areas where the surfacing water table creates waterlogged soils colonized by Amazonian and/or Atlantic arboreal taxa such as Mauritiella aculeata, Acacia, Anadenanthera, Commiphora, Dalbergia, Piptadenia, Poeppigia, Copernicia, Geoffroea, Licania as well as Costus herbs (Andrade-Lima 1981).

MATERIALS AND METHODS
At the Vale do Catimbau peatbog, a peat sediment column of 161,5 cm was retrieved with a Russian sampler (Belokopytov and Beresnevich 1955) in the northern portion of that basin in order to avoid sampling of disturbed sediments previously collected at the center of the bog by Nascimento (2008).At the Bom Jesus site, a 155 cm sediment sequence was obtained with a vibrocore sampler (Martin et al. 1995) in a palm swamp characterized by clayey/organic sediments.
After the opening of the tubes under laboratory conditions, sediments were described and subsampled.A total of 32 samples of one cm 3 were collected along the Catimbau core in various depth intervals (10 cm, 5 cm and 2.5 cm), while in the Bom Jesus sequence, were collected a total of 12 samples, within 10 cm depth intervals.All of them were chemically processed according to the Quaternary Palynology protocol described in Colinvaux et al. (1999): addition of Lycopodium clavatum (exotic marker) spores, followed by HF (hydrofluoric acid) treatment for silicates removal and acetolysis reaction for the destruction of organic matter in the samples as well as within the pollen.Residues were mounted on glycerine and pollen/spores and other palynomorphs were counted under light microscopy.Counts proceeded until a minimum of about 300 pollen grains or 100 to 200 grains for samples with low palynomorph preservation.Pollen sums were calculated based on all pollen taxa present and belonging to the different categories such as arboreal, shrub, terrestrial and aquatic herbaceous pollen grains.Percentage and concentration values of all taxa were calculated by TILIA, TILIAGRAPH software (Grimm and Troostheide 1994), and pollen zones were established by means of a similarity dendrogram by CONISS (Grimm 1987).Accelerated Mass Spectrometry (AMS) radiocarbon dating of selected samples was carried out by Beta Analytics Laboratory, Miami, Florida.All the calibrations were done with the software CALIB 7.1 (Stuiver et al. 2017), using the calibration curve ShCal13 (Hogg et al. 2013) and the age-depth models curves were done by Oxcal 4.3 (Ramsey 2008).

RESULTS
Lithology, radiocarbon dating and sedimentation rates are presented for both sites, followed by separate palynological results.
Peatbog sediments from Catimbau are composed of very dark sandy organic sediments with fine sand laminations at 122.5 cm and from 135 and 140 cm, while the Bom Jesus sequence showed the intercalation of clays and sands (Table I).Radiocarbon ages for the Catimbau sequence vary from 10,322 cal yrs BP to 152 cal yrs BP with dating inversion at around 1,800 cal yrs BP.The Bom Jesus sequence reached 1,749 cal yrs BP at the bottom depth of 100 cm depth, with inversions at ca. 1,700 cal yrs BP, at 154 cm depth (Table II).
The litology and radiocarbon dating results indicate very slow sedimentation rates varying from 0.009 to 0.233 cm year -1 , at Catimbau, as shown on Figure 2, although a significant deposition of 30 cm occurred between the inverted dates of 1,880 and 1,830 cal.yrs BP.The rapid deposition at ca. 1,800 cal.yrs BP at the Catimbau site is probably associated with a humid episode, with torrential rains that could have altered the horizontally layering of the sediments.A hiatus in sedimentation is clear between 6,440 and 1,778 cal.yrs BP in the Catimbau peat core, which appears to bear a correlation to the Mid-Holocene drying climates observed for the Caatinga ecosystem at ca. 4,500 cal.yrs BP.An age model based on previous analyses at the Catimbau site by Nascimento (2008) shows similar results, i.e. low sedimentation rates at ca. 2,000 yrs BP followed  by oscillating values.It is very likely that due to poor radiocarbon control the author was not able to detect the hiatus.On the other hand, deposition at the Bom Jesus site, also presented on Figure 2, reveals age inversions thus not allowing a thorough interpretation of its pollen data.
Figures 3 and 4 show percentage and concentration pollen diagrams for different palynomorph categories, respectively.Percentage and concentration values for each taxon, given between parentheses, refer to the evolution of its representation from bottom to top of each pollen zone.In the palynomorph diagrams, aquatic herbs, pteridophytic and algal spores provide a clear signature of waterlogged conditions for the entire Catimbau deposition.These, in turn, are determined by geological fault lines allowing the surfacing of the underground water table.
CA-1 (160 -111 cm; 10,330 cal.yrs BP -ca.6,000 cal.yrs BP (extrapolated age) is characterized by fl uctuating arboreal elements, both in percentage and concentration values, at the end of this zone.Maximum value of these components occurs at ca. 6,600 cal yrs BP.The most signifi cant taxa, followed by their percent representation from bottom, middle to top of this zone, are represented by Arecaceae (5% -4%), Byrsonima (4.6%-5.4% to 0%), Casearia (1.5% -4.5% to 1.2%), Cecropia (1% -6% to 1.2%), Melastomataceae (1.5% -15% to 1.2%), Myrtaceae (0.7%-2.5% to 2%), Piptadenia (0.5% -1.7% to 0%) and others.Orbignya (Attalea) palm pollen appeared in the record at an extrapolated age of ca.9,000 cal.yrs BP represented by less than 6% of the total pollen sum, whereas Ziziphus,  CA-2 (110 -57 cm; 1,714 -956 cal.yrs BP).The position of sample 110 cm within the contact between reworked and non-reworked layers, as suggested by radiocarbon age inversion, did not allow it to be included in the Ecozone CA-1.It is noteworthy that the CONISS dendrogram separates this sample depth from all previous samples due to reduced similarity between them.CA-2 is subdivided into two sections: a reworked zone from 110 cm to 80 cm interval with date inversion, and a non-disturbed sequence from 79 to 57 cm (1,778 cal.yrs BP to 956 cal.yrs BP).The latter is characterized by the presence of Copaifera (0.75% -4%), together with Arecaceae (5% to 2%), Simarouba (10% to 9%), Celtis (4%) in synchrony with a steady increase in Cecropia (3% to 7%), Melastomataceae (3% to 4%) and Piptadenia (<1% to 7%) while Ziziphus continued its local representation until present conditions fl uctuating within 1% of the pollen sum.Another change in this zone is marked by high Spathiphyllum concentration values, a terrestrial herb found presently at the site in very moist rocky habitats next to the peatbog.In contrast to Spathiphyllum, other herbaceous groups such as Poaceae (Gramineae) and Asteraceae (Compositae) are found in smaller percentage and concentration values.Orbignya (Attalea) palm returns to the Catimbau record at 1,778 cal.yrs BP (80 cm).

DISCUSSION
The Catimbau vegetation during the Holocene can be thought of as a local oasis-like ecosystem within a semi-arid domain maintained by tectonics.Underlain Silurian-Devonian sedimentary rocks function as a source of ground water in a deep aquifer (CPRM 1964) which surfaced possibly due to Mesozoic/Cenozoic geological faulting (Gomes 1995, Nascimento 2008).This geological feature might explain the overall abundance and a prolific representation of Spathiphyllum along the Catimbau record.This herbaceous taxon, restricted to humid rocks and soils (Croat 1988) within semi-arid regions, remained stable in the Catimbau site despite regional climatic fluctuations during the Holocene.
The occurrence of Cecropia in this period is significant.This arboreal taxon, known for its invasive habits and predominance in secondary vegetation, cannot survive in semi-arid climates and can therefore be used as an indicator of successional events under humid climates or in moist soils maintained by edaphic factors (Lorenzi 1998).Humid signals in both sites are given also by the presence of Borreria and Cyperaceae, in association with pteridophytic and algal spores.Borreria, although commonly found in terrestrial settings, has macrophytic species adapted to soils under prolonged flooding such as B. eryngioides, B. quadrifaria (Pott and Pott 1994) , B. saponariifolia and B. capitata (Lorenzi 2000).Likewise, Cyperaceae is a large family comprising ca 4,500 species found mainly in waterlogged conditions (Souza and Lorenzi 2005).
Unlike the Late Pleistocene/Holocene and Early Holocene humid phase displayed by the Icatu record in the Bahian semi-arid Caatinga (De Oliveira et al. 1999), the pollen signal at Catimbau is of very local and not of regional amplitude.This restricted signal is controlled by a peatbog surrounded by a rocky amphitheater, where its abundant pollen signature, maintained by local trees and herbs, masks regional anemophilous sources.However, it is noteworthy the presence of Ziziphus, very likely to be Z.joazeiro, a wellknown evergreen tree species of the xerophyhtic Caatinga of the Sertão region in NE Brazil, thus suggesting nearby semi-arid vegetation around this humid site, throughout the Holocene.
The tendency towards drying of the landscape after the Mid-Holocene shown regionally by the Icatu pollen record (De Oliveira et al. 1999) and by δ 18 O ratios in cave speleothems for NE Brazil (Wang et al. 2004, Cruz et al. 2009, Nace et al. 2014, Mendes 2016), is possibly represented in the Catimbau record by the lack of sediment deposition after ca.6,400 and prior to ca. 1,800 cal yrs BP.This was likely to be a consequence of lowering water tables under a regional and strong semiarid climatic phase.However, it is possible that humid conditions, concomitantly with sediment deposition, persisted during the Mid-Holocene in the climatically closed system of Catimbau, shown by the presence of various moist-adapted taxa either side of the hiatus (ca.6,400 yrs to 1714 cal.yrs BP).This Mid-Holocene climatic phase is possibly correlated with the return of a moist phase in the Early Holocene as detected by Novello et al. (2012), Nace et al. (2014), Mendes (2016).It is possible that the same climatic episode had occurred at Bom Jesus, which could explain the inverted ages as well as the high mineral content (sandy sediments) of its sequence, as well as loss of older deposits.
The increase of Spathiphyllum sp. and sedimentation rates between 2,000 and 1,500 cal yrs BP at Catimbau and Bom Jesus and the date inversions in both sites are suggestive of sediment transport and reworking under intense high energy precipitation.Such storm conditions have been reported for the same time period by Mendes (2016) and Viana et al. (2014).
After ca.500 cal yrs BP towards modern times the pollen signal at the Catimbau site is characterized by a gradual decrease in taxa adapted to high levels of humidity such as aquatic herbs and algae.A semi-arid climatic pattern is also present in the Icatu record where a decline in the arboreal component is in synchrony with an increase of charred particles indicating higher natural or anthropogenic fire frequencies.Both dry scenarios are in agreement with the climatic phases given by Novello (2012), derived from isotopic data in cave speleothems in southern Bahia.These climatic scenarios are well supported by concentration values in the Catimbau pollen signal.One example of this trend is given by Myrtaceae which declined from 1,000 to 350 g/cc, however other elements expected to behave the same way such as Cecropia, Melastomataceae, Arecaceae and Orbignya (Attalea), followed a contrary pathway, and instead increased in the local landscape.The first two taxa are well known elements found in disturbed vegetation (Prather 2014, Shiels and González 2014, Rodríguez-Zorro et al. 2015), whereas the palm family, and particularly Orbignya (Attalea), are important taxa in cultural forests (Balée 2013).
The presence of Orbignya (Attalea) pollen in the Catimbau sediments was already attributed by Nascimento et al. (2009) to human manipulation of the vegetation at around 5,000 yrs BP.Our study shows the occurrence of the Orbignya (Attalea) pollen type as early as 9,000 cal.yrs BP, decreasing during the Mid-Holocene drying event and returning afterwards.Such distribution therefore might suggest Early Holocene human manipulation at the Catimbau site.

CONCLUSIONS
The pollen analysis carried out at the Catimbau site is indicative of an anomalous island of humidity amidst a semi-arid environment created and maintained by geological features such as the Mesozoic/Cenozoic fault systems that permitted the surfacing of ground water.This in turn has allowed this area to function as a refuge for arboreal and herb elements typically found in moist forest ecosystems such as Cecropia, Cedrela, Simarouba, Piptadenia, Melastomataceae, Myrtaceae, Copaifera as well as the conspicuous modern Orbignya (Attalea) palm.
The low geographical amplitude of the pollen signal obtained in this site, contrary to the Icatu sequence which appears to be in synchrony with regional climatic scenarios of NE Brazil, is still useful in determining the presence of nearby Caatinga by the fluctuating presence of Ziziphus pollen in all three zones of the Catimbau record.
The Mid-Holocene drying climatic signal, widely recognized by the Icatu pollen record
a xerophytic Caatinga indicator, fl uctuated in this zone with values under 2%.Concentration values, i.e. number of grains per cubic centimeter of sediment (g/cc), of each taxa, follows the same trend observed in the percentage profi le.These values shows a clear preponderance of humid adapted taxa in the Early Holocene, gradually decreasing in representation towards the Mid Holocene: Arecaceae (15724 -1010 gr/cc), Myrtaceae (1,429 -2,511 to 167), Cecropia (2,859 -5,718 to 502), Melastomataceae (2,654 -10,191 to 606), Moraceae (2,534 -326 to 502), Ziziphus (619 to 167).Pollen grains belonging to Orbignya (Attalea) are represented by 9,911 g/cc and are possibly a consequence of human infl uence on the local landscape.

Figure 2 -Figure 3 -
Figure 2 -Oxcal 4.3 (Ramsey 2008) calibrated age-depth models for the Vale do Catimbau and Bom Jesus peat sequences.The Catimbau data (this study) is given as a continuous line whereas the dotted line represents the age-depth models applied to Nascimento's data (2008).

Figure 4 -
Figure 4 -Concentration pollen diagram of the Vale do Catimbau peat sequence of plant categories (x 1000).Dotted line at the lithology column marks a sedimentary hiatus.

Figure 5 -
Figure 5 -Percentage pollen diagram of the Bom Jesus sediment sequence.

TABLE II Radiocarbon ages for the Catimbau and Bom Jesus samples. Calibrations were calculated by software CALIB 7.1 (Stuiver et al. 2017), ranges of 95,4% (2ᵟ). Final calibrated ages were obtained from the median probability provided of the end calibration.
This sedimentation hiatus is possibly correlated to a phase of generalized drying of that portion of NE Brazil as suggested by the Syntrace Climatic Model (Nace et al. 2014) and by the isotopic data from cave speleothems in State of Rio Grande do Norte (Cruz et al. 2009).In both studies, there is a clear tendency toward arid phases, reaching a peak at ca. 4,000 cal.yrs BP.Under this context the surfacing water table (Costa Filho and Demétrio 2005) at the Catimbau site had been lowered.