LCZO -- Geology, Regolith Survey -- trace and rare earth elements -- Bisley and Icacos -- (2018)

OVERVIEW

Description/Abstract

The thick regolith developed in the humid tropics represents an endmember of critical zone evolution, where shallow and deep biogeochemical cycles can be decoupled in terms of the predominant source of trace elements (atmospheric input at the surface, weathering at depth) and of the processes that control their cycling. To investigate the influence of lithology on trace element behavior and in this potential decoupling, we studied two deep (9.3 and 7.5 m), highly-leached, ridgetop regolith profiles at the Luquillo Critical Zone Observatory, Puerto Rico. These profiles have comparable internal (degree of weathering, topography) and external (vegetation, climate) characteristics, but differ in their underlying bedrock (andesitic volcaniclastic and granitic). At these two sites, we analyzed a large suite of trace elements and used the rare earth elements and yttrium (REY) as tracers of critical zone processes because they are fractionated by the chemical reactions involved in weathering and pedogenesis (e.g., sorption, dissolution, colloidal transport) and by redox fluctuations.

We found that both regolith profiles show atmospheric inputs of trace elements at the surface and evidence of bedrock dissolution at depth, as expected. We also found noticeable differences in the re-distribution of trace elements and REY within the profiles, indicative of different geochemical environments with depth and lithology. In the volcaniclastic profile, trace element and REY behavior is controlled mainly by redox-mediated, sorption/desorption reactions, whereas pH-controlled dissolution/precipitation and sorption reactions predominate in the granitic profile. The most noticeable difference between the two regolith profiles is in the long-term redox conditions, inferred from redox-sensitive elements and Ce anomaly variations, which are more variable and stratified in the volcaniclastic profile and change gradually with depth in the granitic profile. The contrasting redox conditions and the different sources of elements (dust vs. bedrock) produce a decoupling between the surface and deep geochemical environments of the volcaniclastic regolith. The difference in redox conditions between the two lithologies likely stems from the finer grain size and higher clay content of the volcaniclastic regolith.

Creator/Author

Chapela Lara, Maria|Buss, Heather L.|Pett-Ridge, Julie C.

CZOs

Luquillo

Contact

Miguel Leon Miguel.Leon@unh.edu

Subtitle

The effects of lithology on trace element and REE behavior during tropical weathering

SUBJECTS

Disciplines

Geochemistry / Mineralogy

Topics

Geology|Regolith Survey

Subtopic

trace and rare earth elements

Keywords

Rare earth elements|REE|REY|Trace elements|Redox conditions|Tropical weathering|Mass transfer|Critical zone

Variables

Titanium|Zirconium|Hafnium|Thorium|Strontium| Rubidium|Magnesium|Calcium|Manganese|Iron|Vanadium|Chromium|Lead|Uranium|Lanthanum|Cerium|Gadolinium|Ytterbium|Yttrium

Variables ODM2

Titanium|Zirconium|Hafnium|Thorium|Strontium, dissolved|Rubidium|Magnesium|Calcium|Manganese|Iron|Vanadium, total|Chromium, total|Lead, total|Uranium|Lanthanum|Cerium|Ytterbium|Yttrium

TEMPORAL

Date Start

2018-11-01

Date End

2018-11-01

SPATIAL

Field Areas

Bisley|Rio Icacos

Location

Bisley and Icacos

North latitude

18.324044

South latitude

18.323332999999998

West longitude

-65.818056

East longitude

-65.815128

REFERENCE

Citation

Chapela Lara, M., Buss, H.L., Pett-Ridge, J.C. (2018): The effects of lithology on trace element and REE behavior during tropical weathering. Chemical Geology. 500: 88-102. DOI: 10.1016/j.chemgeo.2018.09.024

Publications of this data

Chapela Lara, M., Buss, H.L., Pett-Ridge, J.C. (2018). The effects of lithology on trace element and REE behavior during tropical weathering. Chemical Geology. 500: 88-102 http://dx.doi.org/10.1016/j.chemgeo.2018.09.024

CZO ID

7161