『Abstract
　Sediment fluxes from high standing oceanic islands (HSIs) such as New Zealand are some of the highest known [Milliman J.D. and Syvitski J.P.M.(1992) Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. J. Geol. 100, 525-544]. Recent geochemical work has suggested that along with their extremely high physical weathering yields, many New Zealand watersheds also have very high chemical weathering yields. In New Zealand, the magnitude of both the physical and chemical weathering yields id related to the lithology of the watershed. Most of the previous work on this topic has been undertaken in Southern Alps watersheds of schist and greywacke and in East Cape watersheds of semi-consolidated marine sediments and greywacke. We recently sampled North Island watersheds in the Taranaki and Manawatu-Wanganui regions which have been subjected to volcanism since the Miocene. We sampled watersheds that contain both volcanic and sedimentary rocks. A series of water and sediment samples was collected and analyzed for major, minor and trace elements. This was done to quantify the weathering intensities in the watersheds and to establish the relationship between physical and chemical weathering yields in volcanic lithologies. Our results reveal distinct chemical signatures for the different regions. Waters draining the Taranaki region volcanics are significantly enriched in K⁺, and depleted in Ca²⁺ and Sr²⁺ compared to waters draining the Manawatu-Wanganui region volcanics, which also traverse expanses of sedimentary siltstones and mudstones. The Ca²⁺ and Sr²⁺ depletions may reflect the relative absence of CaCO3 in the Taranaki region watersheds. In addition, sediment samples from the Taranaki region show significant enrichment in Ti, Al, Ca, Fe, Mn, Mg, Ca, and P and depletion in Si and Rb compared to those of the Manawatu-Wanganui region. From total dissolved solids concentrations and mean annual water discharge, we calculate weathering yields of 60-240 tons km^-2 a^-1. These weathering yields fall within the middle to upper range of those previously documented for the Southern Alps (93-480 tons km^-2 a^-1) and East Cape (62-400 tons km^-2 a^-1). Calculated silicate weathering yields is 12-33.6 tons km^-2 a^-1 and CO2 consumption of 852-2390×10³ mol km^-2 a^-1 for the rivers draining the Taranaki volcanic region are higher than those previously reported for watersheds hosted in sedimentary and metamorphosed rock terrains on HSIs. CO2 consumption is found to be within the range previously measured for the basaltic terrains of the Deccan Traps (580-2450×10³ mol km^-2 a^-1) and Reunion（eの頭に´） Island(1300-4400×10³ mol km^-2 a^-1). Our calculated chemical weathering yields demonstrate the importance of HSIs, particularly those with volcanic terrains, when considering global geochemical fluxes.

1. Introduction
2. Study area background
3. Sampling and analytical methods
　3.1. Sample methodology
　3.2. Water analysis
　3.3. Riverbed sediment and rock analysis
　3.4. Data interpretation
4. Results and discussion
　4.1. Solute geochemistry
　　4.1.1. Statistical analysis
　　4.1.2. Plots and molar ratios
　　　4.1.2.1. Na⁺ versus Cl^-
　　　4.1.2.2. Ca²⁺ and Mg²⁺ versus HCO3^-
　　　4.1.2.3. (Na⁺ + K⁺) - Cl^-
　　4.1.3. Total cation yields
　4.2. Streambed sediment and rock geochemistry
　　4.2.1. Statistical analysis
　　4.2.2. Spidergrams
　　4.2.3. Chemical index of alteration
5. Physical and chemical weathering rates
　5.1. Representativeness of the data
　5.2. Physical and chemical weathering yields
　5.3. Silicate weathering and CO2 consumption
6. Conclusions
Acknowledgments
References

Table 6 Physical, chemical and silicate weathering yields and CO2 consumption in New Zealand watersheds and other locations

Watershed	Area upstream of spot sampling location (km2)	Stream Gradient	Physical erosion yield, tons km-2 a-1	Chemical erosion yield, tons km-2 a-1a	Percent chemical H4SiO4 yield, tons km-2 a-1		CO2 flux, ×103 mol km-2 a-1
Taranaki region
Waitara (DS)	705	1.7	1263	113	8	12	852
Manganui (DS)	200	28.6	134	240	64	33.6	2390
Manganui (US)	11	28.6	276	310	53	41.1	2926
Mangamawhete	80	55	205	15	6.8	3.1	217
Waingongoro (US)	47	22	19.3	167	89	25.3	1801
Waingongoro (DS)	202	22.0	39.3	127	76	16.6	1182
Patea (US)	88	6.1	67.9	139	67	23.6	1682
Sedimentary and Ruapehu regions
Waitara (US)	462	1.7	-	95	-	6.5	460
Kai-Iwi	192	4.5	89	60	40	1.8	128
Whanganui	6643	5.2	687	105	13	11.9	851
Whangaehu	1917	10.6	355	121	25	9.2	652
Rabgitikei	2684	4.2	260	131	27	4.9	346
Greywacke and Argillite region
Otaki	305	-	1029	156	13	12.3	875
Other Regions
NZ North Islandb	-	-	50-20,500	62-400	1-53	2.4-15.1	170-1074
NZ South Islandb	-	-	950-32,100	93-480	1-9	4.2-13.3	296-946
Brahmaputrac	-	-	3450	289	8	-	-
Gangac	-	-	2500	245	9	-	-
Congod	-	-	5	8	62	-	51
Andese	-	-	-	-	-	-	220-1000
Himalayase	-	-	-	-	-	-	100-320
Deccan Trapsf	-	-	-	21-63	-	-	580-2540
Reunion Islandg	-	-	1200-9100	63-170	-	-	1300-4400
Martinique and Guadeloupeh	-	-	800-4000	100-120	-	-	1100-1400
a Physical erosion yields are from Hicks and Shankar (2003) unless otherwise noted. b Lyons et al. (2005). c Galy and France-Lanord (2001). d Gaillardet et al. (1995). e Edmond and Huh (1997). f Louvat and Allegre（最初のeの頭に｀） (1997). g Dessert et al. (2001). h Rad et al. (2006).

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