This web page may provide an explanation of the shiny subsurface spherules seen in the trench:

Desert Varnish On Rocks And Boulders.
http://waynesword.palomar.edu/pljan98.htm#Desert

Shiny surfaces had been seen on rocks at the Viking and Pathfinder landing sites and had been attributed to rock varnish, also known as desert varnish. Some scientists believe all rock varnish is biological in origin while others believe it can have a purely chemical origin.
These LPSC reports suggests the hematite deposits seen on Mars could be due to such varnishes and notes in their samples biosignatures were observed:

MICROBIAL FOSSILS DETECTED IN DESERT VARNISH.
B.E. Flood1, C. Allen2 and T. Longazo3, 1Texas A&M
University at Galveston, Texas 77554, marbi2001@yahoo.com, 2 Astromaterials Research and Exploration Science, NASA
Johnson Space Center, Houston, TX 77058, 3Hernandez Engineering, NASA Johnson Space Center, Houston, TX 77058
http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1633.pdf

DESERT VARNISH – PRESERVATION OF BIOFABRICS AND IMPLICATIONS FOR MARS.
Luke W. Probst1, Carlton C. Allen2, Kathie L. Thomas-Keprta3, Simon J. Clemett3, Teresa G. Longazo4, Mayra A.
Nelman-Gonzalez5 , and Clarence Sams2 1Bemidji State University, Bemidji, MN 56601 lwprobst@hotmail.com
2NASA Johnson Space Center, Houston, TX 77058, 3Lockheed Martin Space Operations, Houston, TX 77058,
4Hernandez Engineering, Houston, TX 77062, 5Wyle Life Sciences, Houston, TX 77058.
http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1764.pdf

MAGNETITE IN DESERT VARNISH AND APPLICATIONS TO ROCK VARNISH ON MARS. R. L. Mancinelli1
(rmancinelli@mail.arc.nasa.gov), J. L. Bishop1, S. De2, 1SETI Institute/NASA-ARC, MS 239-4, Moffett Field, CA 94035,
2Exobiology Branch, NASA-ARC, MS 239-4, Moffett Field, CA 94035.
http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1046.pdf

I had thought of the varnish as providing an explanation of the shine in the subsurface spherules, but some examples on the "Desert Varnish On Rocks And Boulders" page may provide an explanation of the spherules themselves:

Crustose Marine Lichens Of The Pacific Coast.

See the close-up image of the lichen Verrucaria maura. The round bodies here look similar to those seen at the Opportunity site.

See also these examples:

Verrucaria maura Wahlenb. (a lichen)
http://www.bioimages.org.uk/HTML/P163349.HTM

Verrucaria maura.
http://www.theseashore.org.uk/theseashore/V.maura%20SRS(US).html

Bob Clark