BIOLOGICAL PROCESSES AND PRESERVATIONAL MODES

Life Strategies and Cycles influence contribution of plant parts in space and time.

r-Strategists

The woody mangrove, Avicennia (black mangrove) is common along the world's coastal zones (mangroves colonize approximately 80% of the world's coastlines). Biomass production in mangrove swamps averages 615 gm/m2/yr, with the release of plant parts physiologically controlled (Lopez-Portillo and Excurra, 1985).

Leaves (83 Wt%) are lost year round; highest loss between April and September (high water stress)
Branch (9 Wt%) loss is related to leaf and flower loss, with greatest quantity lost 45 days after seed release
Aborted flowers and reproductive parts (6 Wt %) with flowering loss peaking in July to September
Seedlings (viviparous) are shed 3-4 months later, peaking in December (when lower salinities occur)

K-strategists

A single, mature deciduous tree (angiosperm) can abscise nearly 100,000 leaves in a single year (K. Johnson, pers. comm. 5/89). With a life expectancy of several centuries, the total number of leaves that could be contributed exceeds several hundred million. Once again, periodicity of plant part loss is governed by life strategy.

Leaf loss can be temporally rhythmic, both in temperate and tropical trees (Ebel and others, 1980).
High biomass loss in evergreen angiosperms is related to the timing and production of new leaves.
Unique growth strategies include masting (cyclical reproductive activity).

Importance of Bauplan

The way in which a plant is constructed (its histological structure) plays a significant role in preservability.

When Hurricane Diana swept through south Florida in the 1960's:

75% of mangroves were defoliated.
marsh grasses (Spartina) remained virtually intact.

BIOCHEMICAL CONSTRAINTS

Physico-chemical attributes of different plant parts are responsible for the diverse rates of decomposition and what remains thereafter.

Labile and easily biodegradable biochemicals are consumed preferentially. Plants constructed of these are rarely preserved. The most resistant biopolymers (see above) and a variety of alteration products remain intact in the fossil record.

Cuticles

Fossilization of Cuticle is NOT a random occurrence, but based upon original biochemistry.

Cutin probably can't be diagenetically altered to cutan because it is found as a distinct monomer in living plants
During biological maturation, the cutin:cutan ratio changes
Diagenetic enrichment (see below) of cutan may occur as a response to chemical alteration operating within mixed polymer cuticles

DIAGENESIS

DIAGENESIS - Processes involving physical and chemical changes in sediment (and that which is incorporated in the sediment) after deposition that converts it to consolidated rock. This includes post-burial, compaction, cementation, and recrystallization (metamorphism).

Burial in and of itself does not prevent continued degradation or further alteration.

Localized geochemical conditions (particularly pore-water Eh/pH relationships) play a major role in "survival"
Changes in water and oxygenation levels accelerate chemical and microbial degradation
Methane and sulfur-reducing bacteria may alter the organic substrate

Modes of Preservation of Plant Parts

Compressions

- The devolatilization of the functional groups comprising the organic compounds results in the concentration of the non-volatile chemicals. Cellulose components are reduced by hydrolysis with an increasing lignin component. Geochemical alteration follows biological alteration. This concentration may result in the coalification of the plant part (depending upon original histological composition).

Impressions

- Complete or nearly complete degradation (often resistant waxy cuticles remain) of the plant part within a clastic matrix may result in the preservation of it's outline (and prominent morphological characteristics).

Adpression

- A term used by Shute and Cleal (1987) to describe a plant fossil specimen showing a mixture of compression (plant parts compressed by sediment where some original or chemically altered plant tissue is still preserved) and impression (an imprint of the fossil plant on sediment or rock surface) states.

Authigenic Cementation

- As devolatilization proceeds, microgeochemical gradients that may be present in fine-grained clastics (particularly reactive clays minerals and available cations in the pore waters) may cause the precipitation of carbonate minerals (calcite, siderite, pistomesite) around the organic matter. The result is the precipitation of a spherical to elliptical concretion around the nucleating organic material.

An example of authigenically preserved plant fossils can be found at MAZON CREEK.

Permineralizations

- Reactive mineral-charged waters may permeate plant cells and react with cell walls. Crystallization of a variety of minerals on the cell walls and within the cell lumens results in the permineralization of the organic matter.

There are several "Petrified Forests" that have been desigated National Treasures in the United States. Visit the Petrified Forest in Arizona as an example.

Duripartic

- When plant parts are able to withstand the degradational processes acting within their final resting area, the resistant plant parts are preserved with little alteration.

Molds & Casts

- When the internal organization of a plant part is degraded resulting in a void, this open area may act as a mold. The mold may be infilled with clastic sediment or crystals. This casting process results in the development of a fossil that represents the internal (and sometimes external) features of the original plant part.

©Copyright 1997 by Robert A. Gastaldo. All rights reserved. No part of these lecture notes may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission from the author.