Work Description

Date: 12 September, 2018

Title: Flowing into the unknown: inferred paleodrainages for studying the ichthyofauna of Brazilian coastal rivers - paleodrainages shapefiles
Authors: Thomaz AT and Knowles LL
Contact: Andrea T Thomaz thomaz@umich.edu

Key Points:
- Paleodrainages during the largest sea-level retreats during the Pleistocene were inferred for the eastern Brazilian coast based on topographic and bathymetric data.
- The study area includes all coastal rivers ranging from Baía de Turiaçu drainage in Maranhão State in the northeast to Laguna dos Patos basin in the board with Uruguay in the south, in Rio Grande do Sul State.

Research Overview:
The eastern coastal basins of Brazil are a series of small and isolated rivers that drain directly into the Atlantic Ocean.
During the Pleistocene, sea-level retreat caused by glaciations exposed the continental shelf, resulting in enlarged paleodrainages that connected rivers that are isolated today.
Using Geographic Information System (GIS), we infer the distribution of these paleodrainages, and their properties for the east Brazilian coast.
Specifically, using elevation/bathymetric data for the largest sea-level retreats during the Pleistocene, the paleodrainages, their area and the number of contemporary basins connected by each palaeodrainage, was inferred.
For the 145 inferred paleodrainages, total paleodrainage area is strongly correlated with the contemporary area encompassed by each paleodrainage, as well as with the number of contemporary basins encompassed by a paleodrainage.
Differences in the continental shelf exposure along the coast affected the degree of past connectivity among contemporary rivers.
With our results freely available, we discuss how paleodrainages have tremendous utility in biological studies, especially in regions with limited geologic data.
With respect to the diverse ichthyofauna of the Brazilian coast, and its high endemism, we highlight how the inferred paleodrainages provide a backdrop to test hypotheses about the effect of past riverine connectivity on diversity patterns.

Methodology:
Paleodrainages and the potential connections between contemporary river basins were estimated with ARCGIS10, using Hydrological tools (v. 10.3).
We reconstructed paleodrainages for a sea level retreat at -125 m.
Topographic and bathymetric information from the General Bathymetric Chart of the Oceans (GEBCO_2014) was used at 30 arc-second resolution (c.1 km; http://www.gebco.net/),
covering from 0 degree latitude north to -40 degrees south, and -30 degrees of longitude east to -60 degrees west.

With a base contour line at -125 m created using the Contour tool to estimate the maximum extend of land exposed during Pleistocene,
the clipped Digital Elevation Model (DEM) raster map was generated. The Fill tool was applied to the map to remove small imperfections,
such as localized depressions in the DEM, which ensures that flow is maintained among neighbouring cells in the map.
Flow direction for each cell in the DEM was determined by its slope using the Flow Direction tool.
Based on this flow direction, ridgelines were identified using the Basin tool, and these ridges delineated the inferred paleodrainages.

To infer the past river connections, a raster containing the total number of cells that drain to each specific cell was generated using the Flow accumulation tool.
Each cell with more than 100 cells draining into it was used to infer the networks of streams by applying the Stream order function.
The riverine raster grids were then converted into vectors using the Stream to Feature tool for generating a map that can be used for analysis of inferred paleodrainages.

*For further information consult Thomaz and Knowles (2018).

Instrument and/or Software specifications:
ARCGIS10, using Hydrological tools (v. 10.3).

Files contained here:
It contains 4 shapefiles:
1) SouthAm = vector of South America
2) Contou125 = vector line delineating the Brazilian coast when sea-level was -125 m below the current sea level.
3) CoastalBasins = polygons with the paleodrainages configuration.
4) stream5EDIT = vector lines showing the potential rivers connectivity paths during the largest sea-level retreats in the Pleistocene.

Related publication(s):
THOMAZ AT, KNOWLES LL (2018) Flowing into unknown territories: inferred paleodrainages for studying the ichthyofauna of Brazilian coastal rivers. Neotropical Ichthyology, 16(3).
THOMAZ AT, MALABARBA LR, KNOWLES LL (2017) Genomic signatures of paleodrainages in a freshwater fish along the southeastern coast of Brazil: genetic structure reflects past riverine properties. Heredity, 119: 287-294.
THOMAZ AT, MALABARBA LR, BONATTO SL, KNOWLES LL (2015) Testing the effect of palaeodrainages versus habitat stability on genetic divergence in riverine systems: study of a Neotropical fish of the Brazilian coastal Atlantic Forest. Journal of Biogeography, 42, 2389-2401.

Use and Access:
This data set is made available under a Attribution-NonCommercial 4.0 International (CC BY-NC 4.0).

To Cite Data:
Thomaz AT and Knowles LL (2018). Flowing into the unknown: inferred paleodrainages for studying the ichthyofauna of Brazilian coastal rivers - paleodrainages shapefiles [Data set]. University of Michigan Deep Blue Data Repository. https://doi.org/10.7302/Z2HH6H8P