Seasonal stable isotope variations of the modern Amazonian freshwater bivalve Anodontites trapesialis

doi:10.1016/S0031-0182(03)00332-8

Palaeogeography, Palaeoclimatology, Palaeoecology

Volume 194, Issue 4, 25 May 2003, Pages 339-354

https://doi.org/10.1016/S0031-0182(03)00332-8 Get rights and content

Abstract

In a floodplain lake of the Amazon River near the city of Iquitos, northeastern Peru, a one-year monitoring experiment was conducted during which water samples and living bivalves (Anodontites trapesialis) were collected with the aim to investigate seasonal δ¹⁸O variation in and fractionation between bivalve aragonite and host water. Both host water and molluscan growth increments show more than 8‰ seasonal variation in δ¹⁸O. In the floodplain lake under study the δ¹⁸O variation of the water is controlled by contrasting dry and wet season evaporation–precipitation regimes. Molluscan δ¹⁸O appears to be in equilibrium with the host water. Although an approximately 4.0‰ offset occurs, δ¹³C records of water and bivalves are in good agreement, suggesting that both δ¹⁸O and δ¹³C of the shells of freshwater bivalve A. trapesialis are good recorders of (palaeo-)environmental conditions. The δ¹³C of Dissolved Inorganic Carbon (DIC) is governed by plant growth and/or by changes in aquatic chemistry, affecting the DIC pool.

Introduction

With approximately 6 000 000 km² the Amazon River has the world’s largest catchment area. The major part of the Amazon Basin belongs to the lower rainforest (80–600 m above sea level) that has little variation in topography (Kalliola and Puhakka, 1993). The Guiana Shield in the north, the Brazilian Shield in the south and the Andes mountain range in the west form the boundaries of the basin. The Amazon River drains the basin to the east into the Atlantic Ocean.

The climatic records of the cities of Iquitos and Pucallpa in Peruvian Amazonia (Fig. 1) show a yearly average temperature of 26°C and relatively humid conditions (80–90%). Pronounced seasonal variation in rainfall leads to the formation of two distinct landscapes in the western part of Amazonia: (1) the terra firme, i.e. land that is never flooded, and (2) the floodplains, which are permanently or temporarily inundated because of seasonal fluctuations in the water level of the main rivers and their tributaries. The floodplains cover ca. 50 000 to 60 000 km² and comprise swamps, lakes and seasonally inundated forests (e.g. Hoorn, 1994).

Near Iquitos the water level of the Amazon River can vary 10 m between wet and dry season (Kalliola and Puhakka, 1993). The wet season in Peruvian Amazonia starts in October and lasts until May, the remaining four months being known as the dry season. Flooding of floodplain lakes (locally called ‘cochas’) may occur some time after the initiation of the wet season. Throughout the dry season the floodplain lakes are separated from the main river channel when the water level of the main rivers drops. During the dry season (June until September), when the floodplain lakes are disconnected from the rivers, their main water sources are direct rainfall and creeks that drain the local basin or terra firme forest.

In general, the δ¹⁸O of rainwater is related to environmental parameters such as latitude, altitude, distance to coast, surface air temperature and the amount of precipitation (Dansgaard, 1964). In the Amazonian hydrological cycle, water vapour is transported from its Atlantic source by the prevailing easterly winds towards the Andes which blocks Pacific water vapour from entering Amazonia (Salati and Vose, 1984). There is a large difference between the wet and dry season δ¹⁸O values of atmospheric water vapour and rainwater, with high values in the dry season and low values in the wet season. According to Grootes et al. (1989) and Grootes (1993), rainfall exceeds evapotranspiration 2–3 times in the wet season resulting in a westward depletion of ¹⁸O in rainwater. Dry season precipitation is recycled by evapotranspiration and therefore not depleted; its values stay close to those of the source (Atlantic Ocean water). However, based on evidence from five selected IAEA/WMO global network stations, Rozanski et al. (1993) conclude that west of Manaus (Brazil) the continental gradient of δ¹⁸O in rainwater is very small or absent in the wet season. They attribute very negative values of δ¹⁸O mostly to the amount effect.

Freshwater unionids often form discrete growth increments in their aragonitic shell (Fig. 2) which can be recognised in cross sections and sampled at high temporal resolution. Dettman and Lohmann (1993), Abell et al. (1995), Jones and Quitmyer (1996), Veinott and Cornett (1998), Dettman et al. (1999), and Wurster and Patterson (2001) showed that seasonal environmental variation is adequately recorded in the isotopic composition of mollusc shells. On the other hand, Fastovsky et al. (1993) argued that some freshwater bivalves precipitate aragonite in disequilibrium with their host water. This was refuted by Dettman et al. (1999) on the argument that Fastovsky and coworkers compared shell δ¹⁸O values to environmental conditions that occurred later than the time of shell growth.

In the present study we monitored a population of Anodontites trapesialis over a 13-month period. These are unionid bivalves that are common in Amazonian floodplain lakes. To determine whether the growth increments of the aragonite shell of A. trapesialis were precipitated in equilibrium with its host water, we analysed the isotopic composition of both aragonite (δ¹⁸O_ar and δ¹³C_ar) and host water (δ¹⁸O_w and δ¹³C_DIC in which DIC stands for Dissolved Inorganic Carbonate). With this experiment we aim to test the applicability of bivalve growth incremental stable isotope profiles for the reconstruction of past Amazonian aquatic environments.

Section snippets

Monitoring project

The studied floodplain lake, locally named ‘Playa Cocha’, lies in an inner bend of the Amazon River, at 3°45.536′S, 73°10.791′W, 5 km southeast of the city of Iquitos, in Peruvian Amazonia (Fig. 1). We monitored a population of living molluscan bivalves (Anodontites trapesialis Lamarck, 1819) during a complete dry–wet season cycle (26 September 1998 until 21 October 1999) in which the water level of the lake changed 5 m. At the beginning of the experiment the size of 24 specimens was measured.

Playa Cocha water

In situ measured water level and bottom water temperature in Playa Cocha varied 5 m and 6°C, respectively, during the monitoring period (Fig. 3a). The lowest water level measured in the lake was 45 cm (15 November 1998) when its size was reduced to about 200 m in length at 5–10 m width. Amazon River water entered Playa Cocha on 26 January 1999. The highest water level was recorded on 8 May 1999 at 547 cm. At the beginning of July the floodplain lake was disconnected again from the river, after

Discussion

This study was performed in order to evaluate how accurate seasonal/climatological signals are recorded in the isotopic geochemistry of bivalve shells. First, we consider the seasonal changes in oxygen and carbon isotopes of Playa Cocha water. Secondly, the accuracy of the Anodontites trapesialis isotope signal in its shell is examined, in relation to fractionation processes. Finally, we discuss the growth rate variations of A. trapesialis.

Conclusions

(1) Pronounced seasonal cyclicity in rainfall patterns leads to strong isotopic variation in the floodplain lakes of the western part of Amazonia. In Playa Cocha, a floodplain lake near the Peruvian city of Iquitos, the oxygen isotope composition of water (δ¹⁸O_w) varied between 0.5‰ and −9.3‰ in the dry season and wet season, respectively. The carbon isotope composition of Playa Cocha water co-varied with δ¹⁸O_w between −1.8‰ in the dry season and −15.3‰ in the wet season. The δ¹⁸O of collected

Acknowledgements

This study was funded by WOTRO, the Netherlands Foundation for the Advancement of Tropical Research, residing under the Netherlands Organization for Scientific Research (NWO). We would like to thank José and Rusbel Arimuya for field assistance, INGEMMET, Peru, for logistic support, Wynanda Koot, Bouk Laçet and V. Wiederhold (Vrije Universiteit, Amsterdam) for preparing the thin sections and Saskia Kars for photographing Anodontites trapesialis. We are indebted to Dr. A. Mackensen from the

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Seasonal stable isotope variations of the modern Amazonian freshwater bivalve Anodontites trapesialis

Abstract

Introduction

Section snippets

Monitoring project

Playa Cocha water

Discussion

Conclusions

Acknowledgements

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Chem. Geol.

Geochim. Cosmochim. Acta

Chem. Geol. (Isotope Geosci. Sec.)

Appl. Geochem.

Geochim. Cosmochim. Acta

The shells of Etheria elliptica as recorders of seasonality at Lake Victoria

Palaeogeogr. Palaeoclimatol. Paleoecol.

Photosynthesis and calcification by Emiliania huxleyi (Prymnesiophyceae) as a function of inorganic carbon species

J. Phycol.

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Science

Floating mussels in the Upper Mississippi River, Minnesota and their implications for dispersal in paleontology and archeology

Central Plains Archeol.

Stable isotopes in precipitation

Tellus

Microsampling carbonates for stable isotope and minor element analysis: physical separation of samples on a 20 micrometer scale

J. Sediment. Res. A