Inclination Shallowing in Sedimentary Rocks: Evidence, Mechanism and Cause - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 37

Fig. 4.2 Change in intensity ( J ) for single-clay synthetic sediments (top) and natural sediments (bottom) as a function of

initial inclination during laboratory compaction. All synthetic sediments include 0.5

Δ

m acicular magnetite and either distilled
pore water (top left) or saline pore water (top right). Montmorillonite, illite, kaolinite and clay-rich natural sediments (Muddy
Natural Sediment) show a strong relationship between initial inclination and the size of the intensity decrease. Decreased clay
content in the natural marine sediment (Silty Marine Sediment) reduces the effect for steep and intermediate inclinations.
(See Colour Plate 4)

μ

function of initial inclination, then the resulting
paleointensity record could have variations that are a
function of PSV inclination variation and not entirely
due to actual variations in the intensity of the geomag-
netic fi eld.
The possibility of a compaction effect on relative
paleointensity records was checked by cross-correlating
the paleointensity and inclination records for DSDP
Site 522 from the South Atlantic Ocean (Tauxe & Hartl
1997). Only for depths between 70 m (7000 cm)

and 100 m (10000 cm) are there consistent anti-
correlations between inclination and paleointensity,
exactly what would be expected for a compaction effect
on sediment magnetic intensity as observed by Deamer
and Kodama in the laboratory (Fig. 4.3). At depths
shallower than 70 m, the porosity of the sediments is
>70% and hence not much volume loss due to compac-
tion has occurred. At 70 m the porosity drops by 15%;
compaction has therefore started to affect the sedi-
ments. The carbonate content of these sediments is