Christina De La Rocha

Enrichment of dissolved silica in the deep equatorial Pacific during the Eocene-Oligocene

Silicon isotope ratios (expressed as δ30Si) in marine microfossils can provide insights into silica cycling over geologic time. Here we used δ30Si of sponge spicules and radiolarian tests from the Paleogene Equatorial Transect (Ocean Drilling Program Leg 199) spanning the Eocene and Oligocene (~50–23xa0Ma) to reconstruct dissolved silica (DSi) concentrations in deep waters and to examine upper ocean δ30Si. The δ30Si values range from −3.16 to +0.18‰ and from −0.07 to +1.42‰ for the sponge and radiolarian records, respectively. Both records show a transition toward lower δ30Si values around 37xa0Ma. The shift in radiolarian δ30Si is interpreted as a consequence of changes in the δ30Si of source DSi to the region. The decrease in sponge δ30Si is interpreted as a transition from low DSi concentrations to higher DSi concentrations, most likely related to the shift toward a solely Southern Ocean source of deep water in the Pacific during the Paleogene that has been suggested by results from paleoceanographic tracers such as neodymium and carbon isotopes. Sponge δ30Si provides relatively direct information about the nutrient content of deep water and is a useful complement to other tracers of deep water circulation in the oceans of the past.

Biosilicification drives a decline of dissolved si in the oceans through geologic time

Biosilicification has driven variation in the global Si cycle over geologic time. The evolution of different eukaryotic lineages that convert dissolved Si (DSi) into mineralized structures (higher plants, siliceous sponges, radiolarians, and diatoms) has driven a secular decrease in DSi in the global ocean leading to the low DSi concentrations seen today. Recent studies, however, have questioned the timing previously proposed for the DSi decreases and the concentration changes through deep time, which would have major implications for the cycling of carbon and other key nutrients in the ocean. Here, we combine relevant genomic data with geological data and present new hypotheses regarding the impact of the evolution of biosilicifying organisms on the DSi inventory of the oceans throughout deep time. Although there is no fossil evidence for true silica biomineralization until the late Precambrian, the timing of the evolution of silica transporter genes suggests that bacterial silicon-related metabolism has been present in the oceans since the Archean with eukaryotic silicon metabolism already occurring in the Neoproterozoic. We hypothesize that biological processes have influenced oceanic DSi concentrations since the beginning of oxygenic photosynthesis. (Less)

Silicon isotope composition of dissolved silica in surface waters of the Elbe Estuary and its tidal marshes

Land ocean silica fluxes pass estuaries. Recent data suggest that the isotopic composition of dissolved silica (DSi) is not altered during this transition. This could have major implication for the oceanic isotopic silicon cycle. To improve our knowledge about isotopic Si cycling in estuaries we investigate the silicon isotopic composition (δ30Si) of DSi of the Elbe Estuary and for the first time of tidal marsh areas. DSi concentrations in the tidal marshes were generally higher during seepage phase than during bulk phase. Negligible tidal variation in δ30Si (1.71xa0±xa00.08–1.87xa0±xa00.13xa0‰) occurred in the freshwater marsh. In the brackish marsh δ30Si was higher during the seepage phase than during the bulk phase, with highest noted values being >2.78xa0±xa00.11xa0‰. In the salt marsh, seepage water had lower δ30Si than bulk water over a total range of 1.81xa0±xa00.03–2.62xa0±xa00.02xa0‰. In the estuary in October, DSi concentrations in the freshwater zone were diminished through removal by diatoms. The δ30Si of DSi increased from 1.43xa0±xa00.11 to 2.33xa0±xa00.08xa0‰. In December, DSi concentrations increased along the estuary through lateral input from tributaries and tidal marshes. δ30Si values in the freshwater and brackish zones were lower than in October. The most notable changes in δ30Si occurred in the tidal freshwater zone of the estuary. This underscores that this zone modulates the delivery of reactive silica from land to sea. If true for other systems estuarine transformation would significantly contribute to the long term control of the silicon isotopic composition of the ocean.

Feeding on dispersed vs. aggregated particles: The effect of zooplankton feeding behavior on vertical flux

Zooplankton feeding activity is hypothesized to attenuate the downward flux of elements in the ocean. We investigated whether the zooplankton community composition could influence the flux attenuation, due to the differences of feeding modes (feeding on dispersed vs. aggregated particles) and of metabolic rates. We fed 5 copepod species—three calanoid, one harpacticoid and one poecilamastoid–microplankton food, in either dispersed or aggregated form and measured rates of respiration, fecal pellet production and egg production. Calanoid copepods were able to feed only on dispersed food; when their food was introduced as aggregates, their pellet production and respiration rates decreased to rates observed for starved individuals. In contrast, harpacticoids and the poecilamastoid copepod Oncaea spp. were able to feed only when the food was in the form of aggregates. The sum of copepod respiration, pellet production and egg production rates was equivalent to a daily minimum carbon demand of ca. 10% body weight-1 for all non-feeding copepods; the carbon demand of calanoids feeding on dispersed food was 2–3 times greater, and the carbon demand of harpacticoids and Oncaea spp. feeding on aggregates was >7 times greater, than the resting rates. The zooplankton species composition combined with the type of available food strongly influences the calculated carbon demand of a copepod community, and thus also the attenuation of vertical carbon flux.

Chicks Need Silica, Too

Wouldn’t you like to have beautiful nails and hair and strong bones? Walk down the supplements aisle of your local drugstore and you might get the idea this takes silica. Buy these capsules, please! Colloidal silica gel, horsetail silica, food grade diatomaceous earth, choline-stabilized orthosilicic acid, monomethyl trisilanol. The variety of supplement silica covers it all from dissolved to colloidal to particulate silica and from inorganic to organic forms of silica. Even if you are a skeptic of supplements, the exuberance of offerings is enough to make you wonder whether indeed our bodies need silica. It’s a strange idea. We’re not like diatoms, glass sponges, choanoflagellates, and many land plants. We don’t make glass intracellularly. And silica, unlike iron and other micronutrients, is not incorporated into any of the major enzymes or reactive proteins whose functioning keeps us alive. So why would we have a nutritional need for silica? And why have the silica supplement hawkers dialed down on a specific target: hair, nails, skin, and bones? What do the supplement makers know that most of us don’t? Or are they just having us on?

Glass Houses and Nanotechnology

Wouldn’t it be cool to live in a world that was full of microscopic houses made of glass? Not just plain glass houses either, but ones with nanoscale details, the minuscule evocations of windows and doors, flagpoles, antennas, and weathervanes. What about living in a world that also boasted bazillions of organisms roaming around with glass skeletons or glass plates of armor and contained land plants woven through with glass shards. But you already know the punchline. There are no what ifs about this. Our world is full of microscopic glass structures wrought in purposeful and epic detail. Thank silica biomineralization. Creatures lurking on almost every major branch of the eukaryotic family tree (sketched in Fig. 5.1) have been producing biominerals of that amorphous, hydrated silica known as opal for as long as there have been such things on Earth as animals, if not for a few hundred million years longer. Today, to name but a bunch, diatoms, sponges, radiolarians, choanoflagellates, chrysophytes, euglyphids, ebridians, heliozoans, thaumatomonads, horsetails, grasses (including rice, wheat, and bamboo), reeds, rushes, sedges, palm trees, forget-me-nots, maize, squashes, bananas, sausage trees, arrowroot, and orchids biomineralize silica and the oceans, rivers, lakes, and their sediments; soils; dust; and many of the fruits, vegetables, and grains we eat abound with this biomineralized opal.

Of Fields, Phytoliths, and Sewage

We hate to be the ones to break it to you, but if lawns can hate, yours hates you. You mow it, probably once a week during the growing season, and then tidy up, conscientiously clearing away the clippings, bagging them up for the garbage truck. Do you know what it is you are doing? Blades of grass are stocked full of silica in the form of phytoliths, those minuscule bits of biogenic silica biomineralized by land plants and introduced briefly in the previous chapter. Every time you throw out grass clippings instead of mulching them, you’re exporting silica from your lawn. Though the silica will be slowly replenished by the weathering of the minerals in the soil under the lawn, but that process can’t compete against even the most mowing-averse, non-mulching lawn tender. Unless you’ve been replenishing it, your lawn may be by now silica-deficient. The same goes for many agricultural fields.

Mystical Crystals of Silica

People split into two different camps—those who believe that crystals have special powers and those who roll their eyes. We (the authors) have long been eye rollers. We are scientists, after all. Acquaintances professing spiritual exuberance for quartz or steeping Himalayan rocks to make crystal energy tea send us into stammers of embarrassment. Crystals are solids surely as mystical as butter. So, alas, the joke was on us when we knuckled down and read up on the scientific behavior of crystalline silica. It’s not exactly as the New Agers and several other more traditional traditions have it, but give a quartz crystal a squeeze and it will give off electricity. Who knew? Physical chemists, physicists, mineral physicists, materials scientists, crystallographers, and engineers, for one (or six) and, hey, now we have a lot of modern technology. Let us explain.

The Making of Humankind: Silica Lends a Hand (and Maybe a Brain)

Not that you’d necessarily think so if you read the Interwebs or watch a modern Hollywood movie, but human beings are smart. Say what you want about the typical intellect of internet comment makers, but even the greatest of the other living great apes couldn’t even dream of being one, much less of being a coder, and what whale or dolphin knows to hope to someday grasp the concept of cinematography, calculus, or the cooking of Italian food. In the midst of even the most chaotic, smog-choked city, with everything it contains and jet planes flying overhead, you can stop and stand bewildered and, hands out, ask: How did we get here? How did we, human beings, get to be smart enough to figure all of this out?

Silica, Be Dammed!

It took us about 180,000 years but finally we did it. We hit Earth’s carrying capacity for hunters and gatherers. That happened more or less 10,000 years ago and in order to keep going forth and multiplying, humanity had to learn how to farm. Talk about a multidisciplinary endeavor. There were plants and animals to be bred, tools to be designed, materials to be discovered, and a whole lot of biology, chemistry, hydrology, geology, meteorology, ecology, and biogeochemistry to be mastered. We’re still working on it (and have added mechanization, transportation, refrigeration, genetic engineering, electronics, and information technology, among other things, to the list). Needless to say, our early stabs at farming were nowhere near as fruitful and reliable nor as intensive and destructive as the farming we do today. But as we slogged through the millennia, growing ever better at farming, ever more of us could be fed. So our numbers kept increasing. Do you see the vicious circle? As long as the human population keeps growing, so must the production of food through farming so that at least some chunk of the population that there has been enough food to produce doesn’t then starve to death. For a long time, much of the getting better at farming meant increasing our control over the landscape and in no small part this was through damming. It also meant increasingly disrupting the biogeochemical cycles of nitrogen and phosphorus in our quest to keep cropland fertilized and productive. Both of these activities have had profound effects on the silica cycle.