Foraminifer Preservation and diagenesis

Foram diagenesis primer

Foraminifer Preservation and diagenesis

Foram diagenesis primer

The diagenetic processes assessing foraminifer preservation and diagenesis include dissolution, overgrowth, recrystallization, infilling, and cementation. Dissolution results in etching of the test surface, removal of material from the inner and outer parts of the test, and formation of cavities in the wall cross-section, and eventually causes fragmentation.

Overgrowth is the growth of new crystals from solution (pore water) and can occur on the inner and outer wall surfaces, and even in cavities. It results in large bladed crystals of typical calcite habit, commonly calcite, oxides, pyrite and dolomite. Recrystallization is a fine scale process by which the original biogenic microgranules are replaced by micron scale crystallites.

Infilling means packing of chamber interiors with sediment (e.g. coccoliths, clay minerals) and / or authigenic minerals. Fine material can consist of coccoliths or other carbonate particles, or clastic material, etc. Cementation is a process that binds of adhering materials to the inner and outer surfaces, usually combines with overgrowth and infilling.

Protocol and qualitative scale for assessing foraminifer preservation 

Foraminifer preservation was assessed shipboard in two stages first by light microscope and then by scanning electron microscope (SEM). The foraminifer assemblage was examined by LM and photographed to assess the extent of fragmentation and staining and whether they are best described as “glassy” or opaque. Three planktonic species Trilobatus trilobus specimen and three benthic species Cibidoides wullerstofi specimen were selected for the assessment. Selected specimens were then imaged as whole tests under SEM umbilical side upward. The outer wall surface were imaged with high magnification images (x 2000), examined additional features such as spine holes, pustules, etc. Specimens were removed from the SEM, gently crushed under a glass slide, and re-imaged at high magnification (x 2000) to investigate the inner wall (focusing on evidence for internal overgrowth and cementation) and wall cross-section on fortuitous breaks, searching for original microgranules or diagenetic crystallites.

A simple qualitative scale was employed for fast assessment and capture of data shipboard, which focuses first on recrystallization, and then on the other diagenetic processes. The classification was evaluated as follows. 

Excellent preservation means most specimens having a “glassy” appearance in light microscope and little evidence of recrystallization in SEM, and little evidence of dissolution, overgrowth, infilling or cementation. 

Very good preservation refers to most specimens recrystallized and opaque but little evidence of dissolution, overgrowth, infilling or cementation; apertures empty. Some fragmentation may occur. 

Good preservation indicates some dissolution, overgrowth, infilling or cementation and apertures may be partly obscured, and fragmentation may be frequent. 

Moderate preservation represents that tests generally were infilled with overgrowth or indurated cement, apertures clogged, and fragmentation may be common.

The examples on the previous page show “Excellent-Very good” and  “Moderate-Poor” preservation

The left hand column of photos: Excellent- very good preservation:

Sample (2.98 m; latest Pleistocene, approximately 0.01 Ma) shows excellent preservation. Foraminifers have a glassy appearance in the light microscope. In cross section, the wall of Trilobatus trilobus shows the platy microgranules typical of the species, and in some specimens the primary organic membrane, spines embedded in the wall, and gametogenic crust were observed.

The right hand column of photos: Good-Moderate preservation:

Sample (530.35m, late Miocene, approximately 10.9 Ma) shows moderate to poor preservation. Foraminifers are generally opaque in the light microscope and strongly overgrown and cemented. Chambers are often filled with pyrite, calcite and/or dolomite. Even so the wall interiors are generally well-preserved, showing microgranular textures, but with evidence of patchy recrystallization into solid patches.