Carbon-13 variation with depth in soils of Brazil and climate change during the Quaternary

Paleoecological and geomorphological studies indicate that, during the middle Holocene, there was a predominance of drier conditions with grassy savannahs replacing forests across the South American continent. Modern savannahs are composed mainly of C4 plants and soils developed under this type of vegetation show enrichment in 13C compared to soils under C3 vegetation cover. If soils contain stabilized organic matter formed in the middle Holocene, we hypothesize that former C4 vegetation would be evidenced by a large enrichment of 13C in soil organic matter (SOM). We investigate this possibility examining the depth variation of carbon isotopic composition in 21 soil profiles collected by different researchers at 14 different sites in Brazil. Of these, profiles from only three sites showed a marked increase of 13C with depth (9–10‰ enrichment in δ13C difference between the surface soil and deepest depth); two sites showed intermediate enrichment (4–5‰), and nine sites showed a small enrichment of approximatelly 2.5‰. The majority of sites showing all-C3 derived SOM were in the Amazon region. Possible causes for the absence of a large 13C enrichment with depth are: (1) dominance of C3 rather than C4 grasses in mid-Holocene savannahas, (2) soil profiles did not preserve organic matter derived from mid-Holocene plants, (3) the retreat of forest areas did not occur on a regional scale, but was a much more localized phenomenon.

possibility examining the depth variation of carbon isotopic composition in 21 soil profiles collected by different researchers at 14 different sites in Brazi I. Of these, profiles from only three sites showed a marked increase of nc with depth (9-10%0 enrichment in one difference between the surface soil and deepest depth); two sites showed intermediate enrichment (4-5%0), and nine sites showed a small enrichment of approximatelly 2.5%0. The majority of sites showing all-C3 derived SOM were in the Amazon region. Possible causes for the absence of a large I 3C enrichment with depth are: ( I) dominance of C3 rather than C4 grasses in mid-Holocene savannahas, (2) soil profiles did not preserve organic matter derived from mid-Holocene plants, (3) the retreat of forest areas did not occur on a regional scale, but was a much more localized phenomenon.

Introduction
Soil organic matter (SOM) is a complex mixture of compounds from different pools, ranging from very labile compounds with very fast cycling times to refractory components that accumulate over millenia (Trumbore 1993). These attributes are susceptible to climate change. which can cause alterations in organic matter inputs and rates of decomposition (Jenkinson et al. 1991 ). The cumulative response of SOM behavior to past and future changes is a significant factor in the global carbon cycle (Prentice and Fung 1990;Tans et al. 1990;Jenkinson et al. 1991).
Paleoecological and geomorphological studies suggest the occurrence of severe climatic changes in the South American continent. It has been hypothesized that there were drier periods during the Pleistocene and Holocene than the present, when the tropical forest was replaced by savannah-like vegetation, with predominance of grasses (Van der Hammen I 974; Absy and Van der Hammen I 976; Absy 1980; Ab ' Saber 1982;B igarella and Andrade-Lima 1982;Leyden 1985;Markgraf 1989;Markgraf 1991;Absy el al. 1991 ). The maximum in the proportion of grass pollen was found in the middle Holocene from ca. 6,000 to 4,000 years before present (BP) for several placed in South America (Absy 1980;Servant et al. 1989;Markgraf 1989;Absy et al. 1991;Ledru 1992Ledru , 1993Servant et al. 1993), from 10.500 to 10,400 years BP for central Brazil (Servant et al. 1989(Servant et al. , 1993Ledru 1993), and ca. 13.000 years BP for eastern Amazonia (Absy et al. 1991).
If these grasses were of the C4 type. it may be possible to find a residual isotopic signal preserved in soil profiles. Depending on the degree of 13 C enrichment in SOM with depth, two basic processes could explain the observed trends I. If the 13C enrichment with depth is small, the decomposition of organic matter, which favors 12 C, is most likely the cause of the trend.
2. If the DC enrichment with depth is large, it is a stronger indication that the signal is due to the previous existence of 13C-enriched vegetation, probably C4 grasses.
We explore those possibilities through the analyses of the stable carbon isotope composition of 21 soil profiles collected at 14 sites in Brazil, encompassing different soil types and climatic conditions (Volkoff et al. 1982;Desjardins et al. 199 1;Valencia 1993;Pessenda et al. 1995;Trumbore et al. 1995). Radiocarbon data for the organic matter from the same soil profiles provide an estimate of the minimum age of stabilized C4 organic matter.

Methods
Locations of soil profiles are shown in Fig. I. A total of 21 soil profiles at 14 sites, encompassing different climate regimes and soil types were collected (Table I). Of these sites 7 were located in OECOLOGIA 106 ( 1996) <D Springer-Verlag 377 the Amazon basin, which is characterized by high temperature and heavy rainfall ( Fig. I}. Three other profiles were also collected in areas of high average temperatures: Juacema, Nhecolandia and Salitre. The first one is located in the northeast region of Brazil, which is mainly characterized by low rainfall (Fig. I). The Nhecolandia profile was collected in the Brazilian Pantanal. one of the largest floodplains in the world, and the Salitre profile was collected in the southermost region of Brazil (Fig. I). rn areas of lower average temperatures, six profiles in four different places (Sao Roque. Piracicaba, Londrina and Tunas) were collected (Fig. I ).
Soil types differed among places ( Table I). The most common soil types in Brazil. Latossolo and Podzolico, which are equivalent to ultisols and oxisols in the American classification system, were present at most sampling sites (Table I). Soils classified as Terra Roxa Estruturada (equivalent to alfisols) were present at two sites, and finally Cambissolo and Areia Quartzoza, both equivalent to inceptisols, were present at one site each. In all sampling sites the vegetation was the primary forest characteristic of each region. Details of soil sampling and soil characteristics can be found elsewhere (Cerri 1979;Volkoff et al. 1982;Soubies and Chauvel 1985;Martins et al. 1991;Higa 1989;Rocha 1990;Valencia 1993;Pessenda et al. 1996;Trumbore et al. 1995 gominas and these analyses were conducted by Valencia ( 1993) for the first three profiles, and by Pessenda et al. (1996), Victoria et al. (1995), a nd Trumbore et al. ( 1995) for the three last profiles, respectively.

Results and discussion
The contemporary vegetation cover of all sites was primary forest (C3 plant type), and the ()1 3C of the surface soil organic matter varies from -28.5 to 26.0%0. In order to make the comparison among different profiles easier, the depth variability of the carbon isotopic composition in each profile was expressed as the difference of the ()13C of a soil depth in relation to the ()1 3C value of the most superficial sampling depth. We call this difference d 13C, defined as: Most of the soil profiles with the smallest d 13 C values ( Fig. 2A and 2B) are from the Amazon Basin (Fig. 1). In these profiles the highest £\l3C value was 3.5%0, but most of the values were smaller than 2.5%0, which indicates that the major cause of 13C enrichment with depth was probably fractionation during decomposition of SOM. The only exception to this pattern was the depth variability observed in the Terra Nova profile, where the ~IJC  (Fig. 20). Profiles from Piracicaba and Londrina showed similar 1 3C enrichment (Fig. 1 ). The profile Lon-I at Londrina reached a ~J3C value of 11.5%0 at the bottom (Fig. 20).
In such cases the most likely cause for this large J3C enrichment with depth is the existence of prior C4 vegetation (Dzurec et al. 1985;Mondenesi et al. 1986;Schwartz et al. 1987;Volkoff and Cerri 1987;Martin et al. 1990;Desjardins et al. 1991;McPherson et al. 1993;Wang et al. 1993;Mariotti and Peterschmitt 1994;Victoria et al. 1995 Pessenda et al. ( 1995) the cause of the increse of 6 13C with depth at these sites is fractionation of isotopes during decomposition and not a past vegetation change. However, palynological evidence of a past vegetation change in favor of grasses that may have been C4 (Ledru 1993), and the 6'3C value of 4.0-6.0%0, suggest that paleovegetation change cannot be ruled out as an explanation. Table 2 summarizes the radiocarbon ages of soil profil es for those where such data were available. The 14C  Table I ages reported represent the average "age" of a carbon atom in SOM, and certainly represent a mixture of both older and younger material. For samples from deep in the soil, the radiocarbon age may be taken as a minimum age for stabilized organic matter. For instance, although the 14C age of organic matter at 100 cm depth in Paragominas soils is about 14,000 years BP, it was estimated by Trumbore et al. (1995) that this average age represents a mixture of 10% modern carbon with 90% radiocarbon-free carbon (i.e., 90% with average age >40,000 years BP).
One the most widely accepted climatic changes in the pat is the drought that occurred in the middle Holocene (Servant et al. 1993). Pollen records from Salitre (Ledru 1993) and Southern Serra dos Carajas, near Altamira (Absy et al. 1991) show sharp increase in grass species. The Piracicaba, Londrina and Terra Nova profiles clearly show a sharp increase in ~13C (Fig. 2C). The deepest dated depths ( 170-180 cm) in the Piracicaba and Lond-rina profiles reach ages of ca. 3600 years BP and ca. 9300 years BP. These soils were presumably subjected to paleoclimatic and paleovegetation changes associated with the mid-Holocene. In Salitre the deepest dated depth (190-200 cm) is almost 7000 years BP, which means that these profiles also experienced the middle Holocene drought. However, the increase of dt3C with depth was not as sharp as in the Piracicaba and Londrina profiles, making it difficult to establish the cause for this increase. No other profiles showed the effects of the middle Holocene dryness in their carbon isotopic composition. In addition, seven other profiles collected in several areas of the Amazon region down to 4 m depth show no significant variation in carbon stable isotopic composition (Sanaiotti, unpublished work). As modern savannah soils clearly show the presence of C4 grasses in their SOM (Dzurec et al. 1985;Mondenesi et al. 1982;Schwartz et al. 1987;Volkoff and Cerri 1987;Martin et al. 1990;Desjardins et al. 1991;McPherson et al. 1993;Wang et al. 1993;Mariotti and Peterschmitt 1994), the apparent absence of C4 signal in majority of the profiles is intriguing.
There are several hypothesis that can explain the lack of a C4 signal. 380 OECOLOGI A 106 ( 1996) Boulet et al. ( 1995) I. The lack of signal would be consistent with dominance by C3 rather than C4 grasses. The same hypothesis was raised by Guillet et al. ( 1988), who found a similar situation in the tropical region of Colombia. However, judging by the modern savannahs of the world, this is unlikely.
2. The retreat of forest areas was not on a regional scale, but was a much more localized phenomenom.
3. The C4 plants were not present long enough to leave their isotopic imprint on stabilized soil organic matter.
The refuge theory for the high degree of biodiversity in tropical Sough America suggests that the high diversity resulted from differential species evolution in forest patches that were isolated when forests were replaced with grassy savannahs during the Pleistocene and Holocene, and remained isolated until a new climatic change provoked the return of an intact continuous forest (Haffer 1969;Vanzolini 1970;Prance 1973Prance , 1982Brown 1974). If the fi ndings of this study are confirmed in further analysis, it would be a strong indication that extensive areas of grass never existed in the Amazon, especially during the Holocene. In view of refuge theory this would have strong implications for interpretations of species evolution of the Amazon rain forest.