Calcite Aragonite

Vaterite

Solid Phases

• Calcite - most common and stable

• Aragonite - often kinetically favored

• Vaterite - rare in nature

Potential Causes of Whitings

• Increased temperature

• Increased Ca2+ and/or CO32-

• phytoplankton blooms

• combination of all of above

Background: Nucleation

• Homogeneous nucleation - from solution

• Heterogeneous Nucleation - on a substrate

• Induction time

Background: Saturation State

• Often denoted by

• = (aCa2+aCO32-)/Karagonite

• If for a mineral in solution, precipitation is favored

We know increasing temperature and nutrient concentrations enhances

aragonite growth.

However, it is much less clear whether phytoplankton act as nucleation "seeds".

Nodularia spumigena 10m

Simple Nucleation Experiments

• Add excess Ca2+ to Pyramid Lake

water until whiting is observed.

• Quantify how much Ca2+ is required

for given conditions

Equipment Required

• Collection bottle• Filtration apparatus• Clock or watch• Stir plate• Burette

• Glass flask

• 0.1 M CaCl2 solution

• Thermometer• Notebook and pen• pH meter (optional)

Variables to Keep Constant

• Temperature

• Stir rate

• Interval of Ca2+ additions

• Particulates in solution

Procedure

• Add drops of 0.1M CaCl2 solution in predetermined time intervals

• Record time and pH after each addition

• Stop when whiting is observed

Data

• The main variable of interest is the amount of titrant required to produce the whiting event.

pH

8.60

8.65

8.70

8.75

8.80

8.85

8.90

8.95

9.00

9.05

9.10

9:21:36 9:36:00 9:50:24 10:04:48 10:19:12

mL

0.00

0.50

1.00

1.50

2.00

9:21:36 9:36:00 9:50:24 10:04:48 10:19:12

Time

mL

tit

ran

t

whiting observed

whiting observed

aragonitegrowth

Results: Nucleation with and without phytoplankton

EXP ID description drop rate mL titrant Final CCPL-4 none 1/15s 2.2 99CCPL-5 w/phyptn 1/15s 2.0 92CCPL-6 none 1/15s 2.4 107CCPL-7 w/phyptn 1/15s 2.2 92

Aragonite nucleates in waters with phytoplankton more readily

Results: Nucleation with and without filtering

EXP ID description drop rate mL titrant final MMR-2 unfiltered 1/7s 1.6 77MMR-3 unfiltered 1/7s 1.5 73MMR-4 filtered 1/7s 1.8 85

EXP ID description drop rate mL titrant final CCPL-2 unfiltered 1/15s 2.0 92CCPL-1 filtered 1/15s 2.1 95

Aragonite nucleates in unfiltered waters more readily

SEM Imaging

• Aragonite crystals look similar in experiments performed with and without added phytoplankton.

control experimentnucleated in the

presenceof nodularia

Discussion/Conclusions

• The amount of added Ca2+ required for nucleation was reproducible in control exps.

• Filtering appears to have an effect on nucleation

• Nucleation occurs more readily in the presence of phytoplankton; suggesting that algal blooms may enhance whiting events.

General implications for natural systems

• Natural waters are commonly supersaturated with respect to CaCO3 minerals, but the concentration and chemical nature of natural inhibitors will determine whether, and at what rate crystal growth actually occurs.

• Substances that enhance nucleation of crystals do not necessarily affect mechanisms of growth, as these are two very different processes.